 preface of the Romance of Modern Chemistry. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. The Romance of Modern Chemistry by James C. Phillip. Preface. Probably most people, when they think of chemistry, suppose that its fascination and its practical bearings can be appreciated only by those who have access to some sequestered laboratory, the doors of which are closed to the uninitiated. This is a mistaken view, for in countless wonderful ways, unknown to the general reader, chemical science is supplying the ordinary needs and contributing to the conveniences of modern life. In the present volume an attempt has been made to deal with this aspect of the subject, and the points of view adopted are different from those of the ordinary textbook. The author ventures to think that those readers who have no technical knowledge may be convinced that radium and other things which figure so largely in the newspapers are by no means the only scientific matters of thrilling interest, and perhaps even readers who are already familiar with the elements of the science may be helped to see afresh the many unsuspected and marvelous ways in which chemical forces are at work beneath our very eyes. The chapters are so arranged that the reader who takes them in order will understand what is brought before him much more easily than if he were to pick out subjects here and there. Only by such consecutive reading is it possible for him to secure the maximum of interest and instruction. CHAPTER I In this 20th century chemistry has become a veritable storehouse of wonder, a cavern of marvel and mystery. Many generations of scientific workers have done their share in the exploration of the cavern, and in the discovery of keys to its innumerable chambers, but much that is obscure or unknown remains. Today there are more explorers than at any previous time, more eager spirits than ever seeking to gain an entrance into those chambers which have not yet yielded up their secrets. Ever and anon some worker, more ingenious or more fortunate than his fellows, makes a notable advance, and his name is on everyone's lips. But all the time, unthought of by the outside world, the rank and file of the explorers is steadily pushing forward and conquering nature's mysteries for the ultimate service of man. In this volume we shall take a peep into some of the chambers which the workers of the past have opened up to us, as well as into some of those which are still partly unexplored. We shall see how the subtle chemical forces which are at work all around us have been revealed and harnessed for the use of man, and how order has been introduced into the apparently hopeless confusion of chemical phenomena. It is not easy to say definitely where and when man first began to grope after the knowledge of chemistry. Of all the ancient nations, the Egyptians seemed to have been the most prominent in this respect. Their knowledge, however, was not acquired in any systematic way, but was rather the result of chance observation. By comparison with the store which has been accumulated in the intervening centuries, the chemical knowledge of the ancients was a negligible quantity. They stood merely on the threshold of the storehouse, little dreaming of the spacious chambers into which succeeding generations were to find their way. Suppose we consider for a moment what actually was the sort of chemical knowledge possessed by the nations of antiquity. They were acquainted with seven metals, namely gold, silver, copper, tin, iron, lead, and quick silver. And although some of these, gold, silver, and copper to a smaller extent, are found as such in nature, the others would have to be extracted from their ores. The ancients must therefore have been familiar with the metallurgical processes necessary for this purpose. It was not long before these seven metals became associated with the sun, moon, and the then known planets, each metal receiving the name and symbol of one heavenly body as shown below, gold, the sun, silver, the moon, quick silver, mercury, copper, venus, tin, Jupiter, iron, Mars, and lead, Saturn. This method of representing the metals by symbols survived till the Middle Ages, and in old prints one may see a flask or bottle within which is sketched a representation of the sun. This is to be taken as indicating that the flask or bottle contains a solution of gold. Besides metallurgical operations, the processes of soap and glass manufacture, of pottery making, and of dyeing were known and practiced in ancient times. Such substances as lime, acetic acid, sugar, soda, potash, alum, and oil of turpentine were in frequent use. The manufacturing processes just mentioned are all essentially chemical, but they were carried out merely by rule of thumb and not on any scientific plan. This is not to be wondered at, for the practical operations were in the hands of artisans alone, and it was not the correct thing for the philosophers of the ancient world to bring their wisdom to bear on arts and crafts. There was in fact a complete divorce between the practical and the theoretical, and therefore no real science. The educated people did not come into touch with the experimental facts on which alone a science could be soundly based. The proper sphere of philosophers was considered to be speculation pure and simple, and to such purpose did they speculate on casual observations that the most grotesque theories were evolved, quite out of harmony with actual facts. An instance of the sort of thing to which this purely speculative science led is furnished by an argument of the eminent philosopher Aristotle. As a result of some of his speculations, he came to the conclusion that a vessel filled with ashes would contain as much water as one of the same size which has no ashes in it, but there was absolutely no desire to see whether this was actually the case or not. These philosophers in fact stood on the threshold of nature's storehouse, endeavouring to predict what should be found within but never making any attempt to effect an entrance and see how the facts squared with their predictions. In one respect, however, the chemical speculations of the ancient philosophers demand some attention, and that is in regard to the ultimate constituents of the visible world. There were supposed to be four primitive independent substances or elements, namely fire, air, earth and water, by the combination of which in different proportions the most varied products could be obtained. According to empeticles, for example, flesh and blood consist of equal parts of all four elements, while bones are one half fire, one quarter earth, and one quarter water. The word element applied to these primitive independent substances has scarcely the same meaning as that which we nowadays attach to it, and indeed Aristotle regarded fire, air, earth, and water as the manifestations of different properties carried about by one and the same kind of matter. The four adjectives warm, cold, dry, and moist describe the fundamental qualities which he supposed to be associated with this primordial matter, and to each of the four elements were assigned two of these properties. Air was represented as warm and moist, water was moist and cold, earth was cold and dry, fire was dry and warm. All this seems very fantastic, but it was a way of looking at things that was current for a long time after Aristotle, indeed down to comparatively recent times. We may well wonder whether the views which we hold about the origin and composition of the natural world will be thought equally fantastic by our scientific descendants. End of chapter one. Chapter two of the Romance of Modern Chemistry. This LibriVox recording is in the public domain. The Romance of Modern Chemistry by James C. Phillip. Chapter two, Alchemy and the Philosopher's Stone. In the previous chapter it was suggested that the historical development of chemistry has resembled the gradual exploration of a cavern full of wonder and of treasure. The reader must not suppose, however, that the progress of the exploration has at all times been equally rapid and equally important. On the contrary, there have been centuries during which chemists contributed very little to the real advance of their science simply because their explorations were carried out under an entirely false guiding principle. This remark applies to the long period in the Middle Ages during which devotion to alchemy was supreme, and although the alchemists in the course of what someone has called their potterings found out many new substances and invented many useful processes, their work was singularly unproductive in the interpretation of chemical phenomena and in the discovery of general principles. They missed the spacious chambers in the cavern because in their blind adherence to the idea that it was possible to convert the baser metals into gold they lost their way in subterranean passages where little treasure was to be found. We have seen that in the nations of antiquity the theoretical and practical sides of natural science were kept absolutely separate. This state of affairs, so disastrous of the advance of true science, was remedied to some extent in the Middle Ages, for the alchemists were not only practical experimenters, but also, many of them at any rate, men of considerable learning and intellectual ability. Unfortunately, however, their chemical theories were based on the fantastic views of the ancient universe, and to these theories the alchemists stuck like limpets to the rock. They had yet to learn that the true method of advance in science is first to study the phenomena and collect the facts, and then build up a theory. The alchemists on the other hand, preferred to start with an a priori theory and then to try to make the facts fit into it. Curiously enough the theory of the transmutation of the metals which dominated the chemistry or rather alchemy of the Middle Ages came in the first place from Arabia. After their conquest of Egypt in the 7th century AD, the Arabians probably absorbed and developed such scientific knowledge as was then in existence, and in any case, the first man, a satisfactory record of whose chemical work has come down to us, was an Arabian, Geber by name. He had a remarkable amount of practical chemical knowledge for that early time, many kinds of apparatus and many laboratory operations, such as distillation, filtration, and crystallization, which are indispensable to every chemist, were familiar to him. Valuable as Geber's practical work was, his theories about the nature of the metals were very wide of the mark. He considered that the metals were all composed of sulfur and mercury. These two substances, or two principles which were embodied in them, were regarded as the parents of all the metals. One metal was supposed to differ from another, only in the proportion of mercury and sulfur which it contained, thus gold was particularly rich in mercury, whereas the common metals had a large proportion of sulfur. On this view it ought to be possible to change one metal into another by merely altering the relative proportion of the two constituents, and the problem of transmuting lead or copper into gold would then be reduced to the discovery of some agent which would withdraw sulfur from the baser metal and add mercury to it. That this way of looking at things should be accepted at all is perhaps not so very strange when we consider what the thinkers of a thousand years ago had inherited from Aristotle and other ancient philosophers. We have seen that Aristotle regarded fire, air, earth, and water as different properties carried by one original kind of matter, and it is not a very big step from this view to the belief that by simply modifying its properties one kind of matter could be converted into another kind. Since water was regarded as moist and cold, while air was moist and warm, it was thought possible by heat alone to convert the second chief property of water into the second chief property of air, that is, it was believed that water could be transformed into air. So we see that the views of Geber and the alchemists who followed him in the Middle Ages were more or less a natural development of the speculations of the ancient philosophers. What is difficult to understand is how the belief in the transmutation of metals continued to dominate the study of chemistry so long as it did, for it was not until the beginning of the 18th century that chemists became generally skeptical about the possibility of converting base metal into gold. For the space of eight centuries or thereabout, the efforts of the great majority of those people who studied chemistry were directed to the discovery of the philosopher's stone, the great elixir which should have the power of changing lead or any other common metal into noble gold. If we are to trust the records that have come down to us, the philosopher's stone was not only sought for but found by a few favored individuals. An eminent physician and chemist Van Helmholt by name, who lived in the 17th century, states that with a small specimen of the philosopher's stone received from an unknown source he had transformed a considerable quantity of mercury into pure gold. A little later a physician in the House Salt of the Prince of Orange published a detailed description of the way in which with the help of a certain preparation he had affected the transmutation of lead into gold. What are we to make of these stories? For no single chemist nowadays believes that anybody ever succeeded in producing so much as one grain of gold from any of the baser metals. The two men whose statements about the production of gold have just been quoted were eminently respectable, and there seems to be no ground whatever for supposing that they wish to deceive their contemporaries or posterity. The only conclusion to which we can come is that they were themselves deceived, that they were the victims of illusion. That seems to be the most charitable explanation. We may however ask the question whether there was anything at all to account for the transmutation of metals being regarded as an incontrovertible fact for so long a period. No doubt the ancient tendency to place more trust in an abstract theory than in any experimental facts had something to do with the persistence of the belief, but in addition certain chemical phenomena were known, which to a superficial observer would seem to show that one metal could be converted into another. For example, there is the experiment which anyone may repeat of putting a piece of iron, such as a steel knife blade, into a solution of blue vitriol or sulfate of copper. However short a time the iron is left in the blue vitriol solution, it comes out exactly like copper with the same characteristic reddish color. This is a very simple straightforward experiment, and to the alchemist it admitted of no other explanation than that the iron had been converted into copper. We now know that no such change takes place. Some copper comes out of the solution and is deposited on the surface of the iron, while by way of holding the balance even an equivalent amount of the iron passes into solution. Other circumstances also favored the postponement of the day when the truth about the transmutation of metals was to be recognized. For one thing the alchemist received valuable support from princes and rulers who were in financial difficulties. It was thought distinctly worthwhile to have a man about cord who might be able to produce gold out of practically nothing. An alchemist were therefore encouraged to continue their search for the philosopher's stone, often at considerable expense to their patrons. This money aspect of the business, as the reader will easily understand, led naturally to all sorts of quacks and charlatans setting up as alchemists and imposing on the credulity or stupidity of princes who were in want of money. Again the air of secrecy which pervaded the alchemist's doings and writings helped to smother the truth. Naturally a man who thought he had discovered the philosopher's stone and could turn lead into gold was very careful not to let his secret get abroad, for if everyone knew the trick the gold would have no more value than the lead out of which it was made. Hence the writings of the alchemists are full of the most unintelligible nonsense that was ever put on paper. Many of them profess to describe their method of preparing the philosopher's stone, but the descriptions consist of nothing but foolish jargon. Perhaps the best way to bring home to the reader the extraordinary character of the alchemistic writings is to quote the following translation from a book on alchemy that appeared in 1608. The philosopher's stone is supposed to be describing itself, quote, I am the old dragon that is present everywhere on the face of the earth. I am father and mother, youthful and ancient, weak and yet more strong, life and death, visible and invisible, hard and soft, descending to the earth and ascending to the heavens, most high and most low, light and heavy. In me the order of nature is oftentimes inverted, in color, number, weights, and measure. I am the carbuncle of the sun, a most noble clarified earth by which thou mayest turn copper, iron, tin and lead into most pure gold, unquote. The philosopher's stone was supposed to possess the most marvelous power. Roger Bacon, one of our own countrymen, declaring that it was able to transform a million times its weight of base metal into gold. Besides this it was supposed to have the power of prolonging life, and was therefore regarded as an elixir vitae. Many other beliefs held at that time were, however, equally absurd. Thus, for example, it was thought that just as an exhausted soil becomes fertile again after a time of rest, so a gold mine which was exhausted would, if left to itself for a long period, again yield abundance of the precious metal. As time went on many chemists, while still adhering to their belief in the transmutation of metals, began to work on other and more useful lines. One school headed by Paracelsus devoted themselves to studying the bearing of chemistry on medicine and made a number of valuable discoveries in this direction. Paracelsus taught that, quote, the object of chemistry is not to make gold but to prepare medicines, unquote, and although this by itself is rather a limited field, it had the effect of gradually drawing men away from the pursuit of alchemy. The way was thus prepared for the rejection of the alchemistic doctrines which had so long rested like a blight on real chemical science. A healthy desire arose to investigate chemical phenomena for the sake of knowledge alone, and it was under these conditions that nature began to reveal her secrets more rapidly. Especially when the explorers discovered the great value of the balance and learned what it had to teach them about the commonplace phenomena of burning, they got back again to the right lines of exploration. From that day to this there has been on the whole steady progress, and it is now our task to look at some of the secret marvels of nature which have been revealed in these last one hundred and fifty years. End of Chapter 2 Chapter 3 of the Romance of Modern Chemistry. This LibriVox recording is in the public domain. The Romance of Modern Chemistry by James C. Philip. Chapter 3 Nature's Building Material A common way of classifying natural objects is suggested by the familiar questions, is it animal, is it vegetable, is it mineral? Now, although from the chemical point of view we are chiefly concerned with so-called dead matter, there are many things belonging to the animal and vegetable kingdoms which we must take into consideration. A certain object may be assigned to one of these two kingdoms, not because it is at present alive, but simply because at one time or another in its history it has been a part of a living thing, a plant, or an animal. A bone, for example, will be considered to belong to the animal kingdom, although in itself it is as dead as a doornail, apart from the living and throbbing body of which it was a member. A tree that refuses to become green under the touch of spring would still be regarded as vegetable, although so far as growth is concerned it might as well be a block of granite. What makes all the difference between the mineral kingdom on the one hand and the animal and vegetable kingdoms on the other hand is the mysterious thing called life, not the mere materials of which the various objects are built up. It is no doubt true that the materials associated with plants and animals and thus involved in the processes of life are frequently of a special kind and this is indicated by describing them as of organic origin in contrast to the inorganic substances which are more especially characteristic of the mineral kingdom. It used to be thought up to about 100 years ago that organic substances could be produced only under the influence of life, but this has been found to be a mistaken view that chemists can produce organic substances in the laboratory starting with inorganic materials and the organic substances so produced are the same in all respects as those formed in the living organism. But however much the chemist may pride himself on his achievements in building up organic substances there is one thing he has not been able to do and that is to produce an organism even of the most elementary kind. Life, which makes all the difference between the organic substances and the organism is apparently beyond the resources of human manufacture. Its origin must be traced to a higher source. A little thought will suffice to remind us of the diverse material used in building up our world both organic and inorganic. Besides the coal and the minerals which we extract from below the crust and the many things which we grow on the surface of this little island we have at our disposal nowadays the products of the ends of the earth in all their variety. But a little simplification may be introduced into this extraordinary diversity when we bear in mind that the chemist has been able to split up most of the complex substances with which we are familiar. He has shown that by various agencies such as for example the action of heat a complex substance may be broken up into simpler substances these latter into still simpler ones and so on. At last we arrive in this way at an irreducible minimum of substances which obstinately refuse to break up into anything simpler and which cannot be converted into each other. These elements as the chemist calls them are so to speak the bricks out of which all known substances are built up. They number about seventy and each kind of brick possesses characteristics which distinguish it from all the other kinds. That being so it is not difficult to understand how the combination of the elements leads to all the infinite variety of nature. For the reader will see at once that if he was provided with seventy kinds of bricks each kind with its own characteristic shade of color and if he was required to put together a structure containing at least two kinds of bricks and up to say any number of bricks of each kind there would be a countless host of products. Now what are these seventy fundamental substances? Many of them are familiar to the reader by name at least. For example lead, sulfur, gold, copper, phosphorus, oxygen, mercury, tin, hydrogen, silver, and carbon. But quite half probably of the elements are unknown even by name to the ordinary individual whilst to the chemist himself they are frequently not much more than names. And this is not to be wondered at for the importance of some of the elements judged by the part they play in the building up of the world and in the service of man is extremely small. Thus Glucinium, Gallium, Scandium, and many others would not be much missed were they to disappear altogether from the family of the elements. Anyone who wants to understand something of the fascinating science of chemistry must be quite clear about the part played by the elements and about the relations in which they stand to the infinite variety of naturally occurring substances. Amongst the elements themselves there is great diversity. Some are gaseous substances like oxygen, hydrogen, nitrogen, chlorine, and helium. Two are liquids under ordinary conditions namely mercury and bromine. While the great majority chiefly metals are solid substances. But this division of the elements into gaseous, liquid, and solid substances is somewhat arbitrary and is valid only for the particular conditions which prevail on our earth. On the heavenly bodies which are much hotter than our planet many of the elements with which we are familiar as solids exist in the gaseous condition. In the extraordinary heat which prevails on the sun even iron is a vapor. It must be borne in mind that the elements are found in nature mostly in some form of mutual combination. Only a few of them occur in the uncombined state or native as it is called. The noble metals and some other elements such as copper, sulfur, oxygen, and nitrogen belong to the latter class. But the minerals composing the great bulk of the earth's crust are combinations of other elements with oxygen and sulfur. The fact that some elements never occur in the native condition becomes intelligible when we make ourselves acquainted with the properties of these elements. Take the case of phosphorus. The chemist has been able by certain subtle processes to extract this element from the ashes of bones. But it has such an aversion to the state of single blessedness that unless precautions are taken to keep it out of contact with air it reverts to the combined state and unites with the oxygen of the atmosphere. It is therefore easily understood why phosphorus is never found native and a similar explanation is forthcoming in the case of other elements. It may occur to the reader to ask is it quite certain that the so-called elements represent the ultimate units of which the natural world is built up? Is it not possible that some substances which are at present regarded as elements may turn out to be combinations of other elements? This is perfectly possible, but not very probable. It is certainly true that water, soda, and potash which up to 100 or 120 years ago were regarded as elements were then found to be really compound substances and it is conceivable that a similar thing might happen again. But it is less likely nowadays. For a substance which has to run the gauntlet of the chemist's modern methods of attack can scarcely pass unscathed unless it is really of an elementary character. On the question of how far the present accepted list of elements is to be regarded as final, the discovery of radium has thrown an interesting and somewhat startling light. For it appears that radium, although an element in the commonly accepted meaning of the word, is undergoing continuous transformation into other elements, the gas helium being one of the products of change. The idea that one element could be transformed into another was cherished by the alchemist, as we have seen, but the whole course of chemical progress in the last century was against the acceptance of that idea, and just as chemists were getting settled in their minds about that important question, radium came along and introduced an air of uncertainty again into the whole business. If it should turn out that one element can actually be converted into another, as radium appears to be changed into helium, there will be some support given to the hypothesis that the elements are simply modifications of one original parent's substance. This plausible suggestion was made long ago and has been revived at occasional intervals, but the evidence of experiment has so far been against its acceptance. In the earlier part of this chapter, the elements have been frequently referred to as existing in a state of combination, in the form of compound substances. Now a compound of two elements is something quite different from a mere mixture. The two elements which combine do so in a very thorough and intimate fashion, with the result that each, as it were, loses its own individuality, and an entirely new individual with other characteristics is produced. The two differently colored bricks, which we may supposed to represent the two elements, are not merely laid side by side so that we could lift the one away from the other without any trouble, but they are fused and coalesced in some mysterious manner into one new brick, different in shape and color from each of the two original ones. The only statement we can make with certainty about the new brick is that its weight is equal to the sum of the weights of the two component bricks. It is very interesting to observe that in some cases we can start with two elements and make either a mixture or a compound of them. Two such elements are iron and sulfur. If the iron is taken in the form of fine filings, which are gray in color, and if these are intimately mixed by grinding with sulfur, which is yellow, a powder is obtained, which is intermediate in color, between gray and yellow. In this mechanical mixture, each component retains its own characteristics just as if the other were not there. The particles of iron can be drawn out of the mixture with a magnet. The particles of sulfur can be dissolved out by using a suitable liquid. The reader will therefore see that it is a comparatively easy matter to separate the components of a mechanical mixture. Suppose now that some of the iron-sulfur mixture is put in a tube and that the tube is heated by a flame at one end. Something of importance obviously takes place, for the contents of the tube above the flame begin to glow vigorously and are raised to a white heat. Even if the tube is no longer heated externally, the flame being removed, the glowing continues until the zone of incandescence has passed right through from one end of the iron-sulfur mixture to the other. This extraordinary display of energy is evidence that the iron-sulfur are combining chemically, and if the product is examined when it has cooled, it will be found that a new substance with entirely different properties has indeed been produced. There are no iron particles now to be attracted by the magnet, and no liquid can be found which will extract the sulfur and leave the iron behind. The iron-sulfur particles are no longer lying side by side. They have united and coalesced to form a compound, sulfite of iron, the properties of which are quite different from those of iron and sulfur. Countless other illustrations might be cited of the fundamental difference between a mere mixture of two elements and a chemical compound of the two. A familiar case is gunpowder. This is a mechanical mixture of sulfur, carbon, and niter, and it is only when the gunpowder is fired that the real chemical process begins. This process results in the production of a number of new substances, gases, absolutely different from the original constituents of the gunpowder. Apart from the thorough going change of properties which accompanies the combination of two elements, chemists have discovered some very remarkable facts bearing on the proportions by weight in which combination takes place. Elements are exceedingly particular as to how far they give themselves away, and nothing will persuade them to go more than a certain distance in meeting the advances of other elements. When iron and sulfur combine, they do so in the proportion of seven parts of iron to four parts of sulfur. If a mixture of eight ounces of iron with four ounces of sulfur were heated, nothing would induce that extra ounce of iron to give up its independence and enter the compound. And similarly, if we took a mixture of seven ounces of iron with five ounces of sulfur, the extra ounce of sulfur would absolutely refuse to be anything else than sulfur. So that elements combine in perfectly definite proportions. However or wherever a compound is produced in the laboratory of the chemist or in the laboratory of nature, it invariably consists of the same elements united in exactly the same proportions. There are cases indeed in which two elements unite to form more than one compound. Thus there are two oxides of copper, one containing eight parts by weight of copper to two parts by weight of oxygen, and another containing eight parts of copper to one of oxygen. Observe that the amount of oxygen uniting with eight parts of copper must be either one or two. No compound can be formed containing between one and two parts of oxygen to eight parts of copper. And this is merely an example of what is always found to be the case. When one element combines with another element to form more than one compound, the amounts of the second element which combined with a definite weight of the first element are as one to two or two to three, some simple ratio of that sort. These remarkable facts about the proportions in which the elements combined were discovered soon after the balance had become part of the regular equipment of a laboratory and chemists began to cast about for an explanation. The result was that they came to regard matter as made up of separate particles of extremely small size, called molecules, which were incapable of further division except by chemical means. A fragment of iron, if magnified sufficiently, would thus resemble a heap of cannonballs, each cannonball representing a molecule. It must be remembered, of course, that this is only a theory, a picture, for nobody has ever divided matter so finely that further division was impossible. A single separate molecule has never been picked out. Indeed, it must be much smaller than anything that has ever been seen, even under the most powerful microscope. Although the molecule of a substance is the smallest particle of that substance which can exist by itself, it is possible to break it up by chemical means. The chemist's experiments have led him to believe that a molecule consists of so-called atoms, sometimes all of one kind, sometimes of different kinds. When the atoms in a molecule are all of the same kind, it is an element which we are considering. When the atoms are of different kinds, it is a compound. To separate the atoms which are present together in any one molecule, we bring another kind of molecule with different atoms alongside. In a great many cases, the atoms will promptly change partners and new molecules, that is, new substances, are produced. Suppose, for example, we bring together a molecule, A, B, containing one atom A and one atom B, and another molecule, Cd, containing one atom C and one atom D. Then a chemical reaction will take place, resulting in the formation of two new molecules, Ac and Bd, or possibly Ad and Bc. This way of picturing the Constitution of Matter enables us to explain the definite proportions in which elements are found to combine. Take the case of copper and oxygen already mentioned. Chemists have come to the conclusion that the atom of copper is four times as heavy as the atom of oxygen. Now, the simplest way in which combination could take place would be by one atom of copper joining with one atom of oxygen to form one particle or molecule, as it is called, of copper oxide. Each molecule therefore of copper oxide would contain four parts by weight of copper to one part of oxygen, or what is the same thing, eight parts by weight of copper to two parts of oxygen. And what has been said of each separate molecule may be said also of the mass of copper oxide, which is simply the sum total of the myriad separate molecules. The proportion of copper to oxygen in the mass of copper oxide would be the same as in each individual molecule. Remembering that the atoms are indivisible, we can easily see that the next simplest ways in which copper could combine with oxygen would be by two atoms of copper joining with one atom of oxygen, or by one atom of copper joining with two atoms of oxygen. The atom of copper being four times as heavy as the atom of oxygen, the first of these two compounds would contain eight parts by weight of copper to one part of oxygen, while the second would contain eight parts by weight of copper to four parts of oxygen. As mentioned above, one of these compounds, the first, has actually been discovered, and it is probable that the second also exists. With the atomic theory of the Constitution of Matter, therefore, we can explain the very notable simplicity and constancy which characterized the manner of combination of the elements. End of Chapter 3. Chapter 4 of the Romance of Modern Chemistry. This LibriVox recording is in the public domain. The Romance of Modern Chemistry by James C. Philip. Chapter 4. Invisible Substances and How We Know of Their Existence. Seeing is believing is a familiar proverb, but we must recognize that the saying does not contain all the truth about the relation of seeing to believing, and that we believe in many things which we cannot see. Even in the realm of matter, a part altogether from the realm of mind, there are some things the existence of which is not directly obvious by the evidence of our senses. The chemist, whose business it is to deal with all sorts and conditions of matter, knows many substances, gases, he calls them, of which he could not say, there, see, smell, touch, taste. A gas may be without smell or taste. It may be as intangible as a spirit, and as for seeing it, why it may be off and away while the observer still thinks he is looking at it. And yet it is possible to satisfy ourselves by some more or less indirect observations that these invisible, odorless, intangible, and tasteless substances do really exist. The chemist at least believes in the existence of gases, such as oxygen, hydrogen, nitrogen, and carbon dioxide, as firmly as he believes in the existence of iron, sulfur, turpentine, or water. It must be observed that the difficulty which is met with, in the case of the four gases just mentioned, does not occur with all gaseous substances. Some betray their presence by their smell. Colgas, for example, is invisible, but fortunately our noses soon warn us when it is out of bounds. Other gases, again, are colored, and we have thus direct evidence of their presence. But there are several indirect ways in which we may convince ourselves that air, oxygen, nitrogen, hydrogen, carbon dioxide, and similar elusive substances have a material existence. Air is indeed invisible, but from the effects which it produces when in motion, we may be pretty sure as to its material nature. In every case where mechanical work is done, we shall find on consideration that the origin of it lies in the motion of some material body, and the wind, which in its destructive mood can lay low a whole forest, can be regularly harnessed to work by the sails of our ships and the arms of our windmills. Since work is done by the wind, there must be a material body moving, and the material body, in this case, is the air. A very good reason for regarding air as a material substance is based on the fact, to which every reader will assent, that two different material bodies cannot occupy a given space at the same time. If we are foolish enough to run against a stone wall, we learn by experience that one material body resents the attempt of another material body to take its place. It offers resistance. Now that is exactly how the air and other gases behave. If a tumbler is inverted in a basin of water, the water does not rise and completely fill the tumbler. There is something inside, which takes up room, and so offers resistance to the water occupying the same space. Reasoning from our general experience of material bodies, we may conclude that the invisible something inside the tumbler is certainly of a material nature. Once again, all solid and liquid substances, with which we are familiar, are characterized as light or heavy. In other words, they have weight. In this respect also, air and other gases conform to what is commonly characteristic of all material bodies. It is true that, bulk for bulk, gases weigh much less than water or wood or stone, but the difference is only one of degree. One simple way of showing that air has weight is to put a little water in a glass flask and let it boil vigorously until the flask is full of steam. It is then corked tightly and removed at once from the source of heat. When the flask and its contents have become quite cold, they are put on one side of a sensitive balance, sufficient weights being put on the other side to keep it level. The cork is then removed for a moment, and it will be observed that the side of the balance on which the flask was placed goes down at once, showing that the mere opening of the flask causes it to become heavier. What has happened is that the steam which filled the flask when it was hot became condensed to water when the flask had cooled, thereby leaving room for air to enter as soon as the cork was removed. It is the entrance of this invisible something from the surrounding atmosphere which makes the flask heavier. Gases then have weight and are on this ground also to be reckoned as material substances. As has been said already, gases are very light compared with other substances. They are matter in a very attenuated form. A pint of water is nearly 800 times as heavy as a pint of air, and with the gas hydrogen the contrast is still more marked. For air is 14 and a half times as heavy as hydrogen, so that a pint of water weighs 11,500 times as much as a pint of hydrogen. It may truly be said that a pint of hydrogen is as light as the proverbial feather, for its weight is only between one five hundredth and one six hundredth of an ounce. Hydrogen is in fact the lightest substance known. The fact that hydrogen is so much lighter than air is of great importance in the manipulation of balloons. In order that a balloon may itself rise in the air and carry as well a load in its car, it must be filled with something which is considerably lighter than air. For this purpose hydrogen is the ideal substance, but coal gas, which contains a good deal of hydrogen, is often employed. Bulk for bulk, coal gas is about half as heavy as air. We have been comparing air with hydrogen, but it is important to bear in mind that whereas hydrogen is an element, air is a mixture chiefly of the two elements, oxygen and nitrogen, in the proportion of one volume of the former to four volumes of the latter. Air is not a chemical compound of oxygen and nitrogen, and from what has been said already about the essential difference between a mechanical mixture and a chemical compound of two elements, it will be understood that the properties of air are a sort of cross between the properties of oxygen and those of nitrogen. Both these gases are without color or smell, but in their chemical behavior they are widely different. Oxygen is a very active element, eager to enter into chemical combination with all sorts of bodies, and its power of supporting life is simply one face of its activity. Nitrogen, on the other hand, is a neutral, sluggish and inert gas without any ambitions in the direction of chemical union. This being so, it is not surprising that air acts like diluted oxygen, the nitrogen as it were, chilling the enthusiasm of the more active gas. Many things burn in air, that is, they combine chemically with the oxygen which it contains, but the combustion is much more vigorous when pure undiluted oxygen is supplied. The process of respiration is very similar to the process of combustion, and it will be remembered that in cases of serious illness, and as a last resort, pure oxygen is sometimes supplied to the patient instead of air in order to support for a little longer the flickering flame of life. A little chip of wood serves very well to show the difference between pure nitrogen, pure oxygen, and the air which is a mixture of both. If the chip is set alight and is then blown out, it continues to glow in the air for some considerable time. When the still glowing chip is thrust into a jar of pure oxygen, it at once bursts into flame, whereas if it were thrust into a jar of pure nitrogen, it would be immediately and completely extinguished. So we learn that of the two chief constituents of atmospheric air, one supports combustion, the other does not. Besides oxygen and nitrogen, there are a number of other gases in the atmosphere, but only in very small amounts. The chief of these are argon, a gas resembling nitrogen, to the extent of nine volumes in one thousand of air, water vapor in varying amount, and carbon dioxide to the extent of three or four volumes in ten thousand. The last mentioned gas is being constantly produced by the combustion of all sorts of fuel and in the respiration of animals, whereas the air which we take into our lungs contains, as has just been said, 0.03 or 0.04 percent of carbon dioxide, expired air contains as much as three to five percent of carbon dioxide and correspondingly less oxygen. Strange to say, this constant enormous production of carbon dioxide does not lead to any increase in the average amount of that gas in the atmosphere, for it is as constantly being removed by the instrumentality of the vegetable world. The green leaves of plants, aided by sunlight, have the power of decomposing carbon dioxide, liberating the oxygen, and using the carbon for their own consumption. In regard, therefore, to the production and consumption of carbon dioxide, the animal and vegetable kingdoms are complementary to each other. To the ordinary person it may appear rather a difficult matter to detect the presence of these odorless, invisible gases, but the chemist has discovered ready methods of recognizing and distinguishing them. The properties of each gas have been carefully studied, and in almost all cases, substances have been found which will behave in some characteristic manner when a particular gas is present and remain unaffected when that gas is absent. One of these useful substances employed to test for the presence of carbon dioxide is lime water. When slaked lime is shaken with water, a little of it dissolves, the water becomes slightly alkaline, and the clear part, free from sediment, is known as lime water. Now when a mixture of gases containing carbon dioxide is shaken with lime water, or is bubbled through the lime water, the latter becomes quite cloudy, owing to the formation of chalk. No other gas behaves towards lime water in this peculiar manner, so that we are able to obtain visible proof of the presence of a gas, which is itself quite invisible. It is sometimes very necessary to be able to detect the presence of carbon dioxide, for although the gas is not actively poisonous, yet it does not support life, and its presence in large quantity is very harmful. In all processes of fermentation, as, for example, in the brewing of beer, large quantities of carbon dioxide are produced. Further, this gas being considerably heavier than air has a habit of accumulating at the bottom of vessels and forming what may be regarded as invisible pools. Hence, it has occasionally happened that a brewery worker descending into one of the large vats for the purpose of cleaning it has collapsed fatally, practically drowned in the carbon dioxide which had collected at the bottom of the vat. When a descent has to be made either into a brewer's vat, or into an old well, where a similar accumulation of carbon dioxide may occur, a lighted candle ought first to be lowered to the bottom. Should the candle continue to burn as brightly as in the open air, no one need hesitate to follow it. If the candle, however, goes out, or even gets dim only, it is evidence that there is a dangerously large quantity of carbon dioxide present. The element carbon combines with oxygen in more than one proportion, giving rise not only to carbon dioxide, but also to carbon monoxide. This latter substance is a colorless and odorless gas, which burns with the blue flame and is intensely poisonous. Anyone who watches a clear coal fire on a winter evening will notice little tongues of blue flame. These are due to carbon monoxide, which readily combines with more oxygen to form carbon dioxide. Carbon monoxide has a curious effect on the blood, an effect which is directly associated with its poisonous properties. It has the power of forming a compound with the hemoglobin, the coloring matter of the blood, and this involves a slight change of tint. By shaking up the suspected gas with a little blood, and then comparing the latter with some of the original blood, either by mere inspection or by means of a spectroscope, one may detect quite small quantities of carbon monoxide. Some very interesting cases are on record in which mice have been used to indicate the presence of carbon monoxide in an atmosphere. Small animals such as mice are affected by this poisonous gas more rapidly than human beings, and the behavior of mice therefore serves to give warning of its presence in dangerous proportions. After the Snaffil Mine Disaster in 1897, for instance, the rescue party, headed by Professor Leneve Foster, descended latter after latter into the shaft only after a mouse had been previously let down to the next lower level. A lighted candle was also attached to the cage containing the mouse. By the aid of this testing apparatus, says Professor Foster in his report, it was easily ascertained without any risk that the air was not bad as far as the 115 fathoms level, and that it became poisonous and deadly at the 130. The mice showed precisely the same symptoms as human beings, for if not completely dead on arriving at the surface they had lost all power in their legs, whilst pinkness in the snout recalled the pink lips of the dead bodies of the unfortunate miners. Until recently it was the regular custom to carry a couple of white mice on every submarine boat, the object being the detection of any carbon monoxide which might be produced by imperfect combustion of the gasoline. It appears, however, that mice are not sufficiently sensitive to small quantities of the gas, and the practice of carrying them on submarines is now quite rare. The question may have occurred to the reader, how does it come about that gases, while obeying the fundamental laws of matter in many respects, are yet so utterly different from the more compact forms of matter with which we are acquainted, namely liquids and solids. It is not only that gases are frequently invisible, but they are peculiar also in their ability to occupy fully any space that is offered to them. If a quantity of gas which fills a 10-gallon gasometer is transferred to one holding 20 gallons, the gas will occupy every corner of the latter. There is, of course, less of it at any particular point, and its total weight remains the same, but its distribution is carried to the utmost limits of the containing vessel, however large that may be. A moment's thought will show how different this is from the case of a liquid such as water. 10 gallons of water remain 10 gallons, whether the containing cistern holds 10, 20, or 100 gallons. Gases then are distinguished from liquids by their remarkable expansibility and compressibility. The space or volume which a given quantity of gas occupies depends altogether on the pressure to which it is subjected. And the very simple law has been discovered that the volume of a gas diminishes in the same proportion as the pressure increases. That is, when the pressure is doubled, the volume is halved. When the pressure is trebled, the volume is one-third of what it was originally, and so on. The volume of a gas is further, very sensitive to changes of temperature, and it has been found that a gas which occupies 10 gallons at 32 degrees Fahrenheit will occupy between 13 and 14 gallons at 212 degrees Fahrenheit, provided the pressure has remained the same. This behavior again is quite different from that exhibited by liquids. Everybody knows that a pint of water does not become noticeably more bulky when it is raised to boiling. As a matter of fact, it does expand, but the expansion is too slight to be detected by the eye. These striking differences between gases and liquids were, of course, obvious to our scientific forefathers, and some generations ago, they adopted the explanation which still holds the field. We first assume the atomic nature of matter. That is, we suppose that if we had a microscope powerful enough, we should find that an apparently continuous and homogeneous piece of matter is really discontinuous, consisting ultimately of tiny, separate, and distinct specks or molecules, just as what looks like a single homogeneous black mound in the distance may turn out on closer inspection to be a heap of separate cannonballs. Thus we suppose further, and this seems very unlikely at first, that in the case of a liquid or a gas, the ultimate particles are in a state of continual motion. The particles or molecules of a gas are to be pictured as rushing hither and thither at a very high speed, constantly colliding with one another and with the walls of the containing vessel. The pressure which the gas exerts on the walls of the containing vessel, a pressure which we may easily measure, is due simply to the impacts delivered by the myriads of moving molecules. Each molecule, as it comes up to the wall of the containing vessel, delivers its blow and rebounds with undiminished speed to continue its zigzag course among the other molecules. If part of the wall of the containing vessel is removed, then the molecules immediately rush ahead and occupy whatever space is offered to the gas. Although the picture just outlined of the conditions which prevail in a gas may seem somewhat improbable to the reader, it has been found capable of giving an excellent interpretation of the varied behavior of gases, but that is another story and would lead us too far. The molecules of a gas are very small compared with the spaces between them. When it is remembered that the molecules in a volume of gas about the size of a pinhead are 30 million times as numerous as the human beings on the face of the globe, it will be seen that a gas molecule is quite the smallest thing we can think of. Not less surprising than the size of the molecules is the rate at which they move. If it came to a race between an express train and a molecule of oxygen, the train would be hopelessly out of it, for the oxygen molecule slips along at the rate of about 20 miles a minute. Now why should liquids be so different from gases? So much more readily visible, so much more tangible, so much less changeable in their volume. The key to the difficulty lies in the recognition that in a liquid the molecules are much closer together than in a gas. Just as one heavenly body attracts another, so a molecule is subject to the attractive force of the surrounding molecules, and it is only because the molecules of a gas are relatively so far away from each other that the attraction may be neglected in this case. In a liquid, however, where the molecules, although still endowed with the power of rapid motion, exert a powerful attraction on each other, it becomes very difficult for an individual molecule to escape through the surface of the liquid. There is, as it were, a social force exerted which seeks to prevent the individual molecule deserting the community. Those molecules which attempt to escape through the surface with a rush and so evaporate have to run the gauntlet of the crowded molecules around, and most of them are prevented. Thus it comes that a liquid is not at liberty to expand to any extent like a gas. A liquid has a definite volume, whereas a gas, like the vicar of Bray, adapts itself to suit the circumstances in quite a remarkable manner. End of Chapter 4 Chapter 5 of the Romance of Modern Chemistry This LibriVox recording is in the public domain. The Romance of Modern Chemistry by James C. Phillips Chapter 5 Elements with a Double Identity In the study of chemistry, one constantly encounters puzzling phenomena, the interpretation of which involves much patient labor on the part of the investigator. One of these puzzling things is the fact that some substances, which are undoubtedly elements, have a way of appearing in different forms, according to the circumstances under which they are produced. We know that an actor plays sometimes one part, sometimes another, but although his get-up differs from time to time, it is always the same man underneath. So an element may be found masquerading in the garb of strange, unwanted properties, which are apt to deceive the onlooker, and it is one of the triumphs of chemical science, that by its penetrating methods it has been able to identify a given kind of matter, however it may be masked. To begin with, it is found that some substances are like the chameleon, which can change the color of its skin, or like the mountain hare, whose fur is brown in summer and white in winter. Such substances exist in two forms of different color. It is not only in regard to color, however, that the two modifications differ. Their other properties are quite distinct also. A good illustration of this is furnished by phosphorus, which was referred to in a previous chapter as one of the elements which occur in nature, always in a state of combination, and never free. Phosphorous, however, can be extracted from bone ash by certain processes, and when prepared in this way, it is a yellowish-white, waxy solid, which can be cut with a knife. It has to be kept underwater, for if exposed to the air, it combines with the oxygen and is gradually converted into a compound of phosphorus and oxygen. Phosphorus is very easily melted, and if the water under which it is kept is raised to a temperature a little above blood heat, the phosphorus becomes a liquid. It takes fire with the greatest readiness, and if, instead of being melted underwater, a piece of phosphorus is heated in the air, it will ignite at a temperature very little above its melting point. In the dark, a lump of phosphorus exhibits a curious glow or phosphorescence, which is directly connected with the action of the oxygen in the air upon it. From what has been said, it will be understood that phosphorus is not dissolved by water, but there is one liquid which dissolves large quantities of the element, namely carbon disulfide. On this property, a pretty experiment can be based, for if a little of the solution of phosphorus and carbon disulfide is poured on blotting paper, the finely divided phosphorus which is left after the evaporation of the carbon disulfide takes fire spontaneously. Ordinary phosphorus is further extremely poisonous, and many cases have occurred in which children have been poisoned by sucking the ends of matches in the preparation of which phosphorus is used. Let the reader now draw a mental picture of this yellowish white waxy solid easily melted and readily set on fire, very poisonous, and giving a characteristic phosphorescence in the dark, and put side by side with it the picture of another substance with the following different characteristics, a red powder which cannot be melted, which can be heated in the open air to a temperature of 450 degrees Fahrenheit without taking fire, which does not phosphoresce in the dark, is not poisonous, and not soluble in carbon disulfide. Nobody looking casually at these two substances would dream of regarding them as anything else than quite separate and distinct, and yet the fact is they are both the same element, phosphorus. The chemist has learned how to convert the ordinary yellow phosphorus into the red, and how the reverse change of the red into the yellow may be affected. Besides, the compounds which are prepared from the red variety are exactly the same as those obtained from the yellow form, so that there is no doubt that phosphorus is an element with a double identity. How is it that a given element is able to assume different characteristics? How is it that such totally distinct properties can be associated with one and the same kind of matter? There are two possible causes for this curious phenomenon, and if we build on the foundation already laid in a previous chapter, we may be able to make the explanation clear. It was said there that the smallest particles of a substance which can exist by themselves are called molecules, each of these containing one or more atoms. In the case of an element, the atoms which go to make up the molecule are certainly all of one kind, but a further question arises about the number. And the first possible cause of the phenomenon that an element exists in two absolutely different forms, like red and yellow phosphorus, is simply this, that the molecules in the two cases contain a different number of atoms. Since we are dealing with one and the same element, the atoms in the two cases must be the same in kind, but there may be more of them in the one case than in the other. There is, however, another possible explanation. We must remember that not only are atoms grouped to form molecules, but molecules are massed together to form the substance as it presents itself to our eyes. If the hordes of molecules are arranged in regular fashion, then we get a crystalline substance. If they are arranged anyhow, we get an amorphous substance, that is one without form. So with phosphorus, the molecules may be marshaled differently in the two varieties. Of the two explanations which may thus be given for the existence of phosphorus in two distinct forms, the latter is the more probable. Red or amorphous phosphorus differs from yellow phosphorus, not in having a different number of atoms in the molecule, but in that the molecules are arranged differently in the two cases. A most interesting and more familiar example of an element occurring in different forms is furnished by carbon. There are no less than three modifications of this element, two of which, at least, are as the poles asunder in respect to outward appearance and commercial value. It is indeed difficult to realize that dull amorphous carbon in the forms of charcoal or lamplac is the same element as the brilliant flashing diamond, yet so it is. While in addition to these two modifications of carbon, there is a third, quite distinct from both, and variously known as graphite, black lead, or plumago. Graphite is a little more distinguished-looking than charcoal, but is mean and commonplace in comparison with the diamond. The three forms of carbon, however, differ not only in outward appearance, but in the value we set on them and in the uses to which they are put. The diamond is very highly prized as a gem and fetches in the market far more than its weight in gold. All real diamonds, which the reader has ever seen, have been obtained from natural sources and diamond mining is a regular form of enterprise. Many attempts have been made in recent times to manufacture diamonds, reminding one of the efforts of the alchemists to convert lead into gold. Reflection, however, shows that these modern attempts are considerably less ambitious. Their aim is not to change one element into another, but to convert one form of a given element into another form. The forms of carbon other than the diamond are easily obtainable, and the endeavor to change some of this plentiful material into a more valuable article is very natural. More than that, the attempt to manufacture diamonds has been actually successful from the scientific and laboratory point of view, although not from a commercial standpoint. Maussan, the French chemist, working on the idea that diamonds are carbon, which has been crystallized under great pressure, dissolved amorphous carbon in a crucible containing molten iron, heated the crucible in the electric furnace, and cooled it suddenly by plunging into molten lead. The temperature of the molten lead is very much lower than that of molten iron, so that the outside portions of the ladder in the crucible solidified immediately. As the iron inside this crust gradually solidified, enormous pressure was produced, for iron, like water, expands when it passes from the liquid to the solid state. The carbon, therefore, which was dissolved in the iron, crystallized out under great pressure. Fragments of diamond were obtained in this way, too small, however, to be of any value as gems. Although it is so difficult a matter to obtain even small diamonds from charcoal or graphite, the reverse change can be quite simply affected. If a diamond is strongly heated, it becomes more bulky and is converted into something that resembles coke or graphite. That is, it loses all the special crystalline character to which the diamond owes its brilliancy. The reader must bear in mind the distinction between artificial and imitation diamonds. Such artificial diamonds, as were made by Maussan, were the real article, and were found to consist of carbon. Imitation diamonds, on the other hand, contain no carbon. They consist of a soft, heavy, flint glass known by the curious name of paste. One interesting way of distinguishing real from imitation diamonds is to bring them close to a little radium salt in a dark room. Under this stimulus, the real diamond phosphoreses, but the imitation article makes no response. The diamond is not only ornamental, it has many practical uses as well. One of the most remarkable things about it is extraordinary hardness, in virtue of which it can scratch even a piece of hardened steel. With a fragment of a diamond fitted into a stem, it is possible to write on glass, as with a pen on paper. And with the natural edge of a small diamond crystal, one can make a cut in a glass plate so that the latter can be broken off like a piece of wood which has been nearly sawn through. The hardness of the diamond accounts also for its great usefulness in rock-boring tools. With a diamond drilled, that is, a steel cylinder round the edge of which is fixed a series of diamonds, the hardest rocks can be gradually pierced. In polishing a diamond, the only material which is of any use is diamond dust. Even emery is too soft to touch it. The phrase diamond cut diamond has its explanation in what has just been said. Graphite or black lead, as it is commonly called, is easily distinguished from the diamond. It is a grayish black substance, crystalline certainly, but soft and soapy to the touch. People give the name of black lead to this form of carbon because they were under the mistaken impression that it contained lead. The power of graphite to give a mark on paper, a property which has found application in the manufacturer of pencils, is exhibited also by metallic lead, hence the really erroneous name of black lead. There are many other ways in which graphite is usefully applied beside the manufacturer of pencils. It is scarcely affected at all by exposure to great heat and is accordingly made up along with clay into crucibles for use at high temperatures. Then it is used to coat ironwork, grates for example, in order to protect it from rusting, and at the same time to give it a shiny appearance. Another curious use to which graphite is put is the lubricating of machinery working at high temperature at which ordinary oil would be unsuitable. Whereas diamond and graphite occur naturally, the various forms of amorphous carbon are generally obtained as the products of human operations, thus wood charcoal is got by the partial combustion of wood. Lamp black is the product of the imperfect combustion of oil, while animal charcoal is obtained by heating bones very strongly. Wood charcoal, lamp black and animal charcoal or bone black all consist of amorphous carbon and are applied in many useful and interesting ways. One of the main characteristics of wood charcoal is its power of absorbing gases in large quantities, a property which renders it of value in the purification of bad air. By passage through charcoal filters, sewer gases and other noxious emanations may be rendered harmless. Bone black again has a remarkable power of removing coloring matter from liquids, such as red wine or indigo solution. It is accordingly employed very extensively in decolorizing sugar during the process of refining. Lamp black on the other hand is applied for quite different purposes. It is useful as an artist's pigment in both oils and watercolors and forms the chief ingredient of Indian ink and printing ink. The uses to which carbon in its various forms may be put are in fact legion and in the face of these it is necessary to reemphasize the fact that diamond, black lead and charcoal are all modifications of this one element. The fundamental experiments on which this statement is based were carried out more than a century ago. Before this people were in great doubt about the exact nature of the diamond, but it was then shown that starting with a given weight of either diamond, graphite or charcoal, one obtained in all three cases the same weight of the gas carbon dioxide and nothing else besides. This experiment proved incontestably that diamond, graphite and charcoal are merely different forms of one and the same element. The explanation which was given for the existence of two modifications of phosphorus is valid also in the case of carbon. Diamond, graphite and charcoal differ not in the number of atoms contained in the molecule, but in the arrangement of the molecules to form the substance as it appears to our eyes. There is however one notable illustration of an element existing in two forms which differ in respect of the number of atoms in the molecule. That is the common element oxygen. The molecule of this gas contains two atoms, but under certain circumstances it is possible to induce three atoms of oxygen to club together in a molecule and then we have ozone. Long before anybody knew about this curious substance, a peculiar smell had been noticed whenever an electrical machine was at work and people adopted what seemed the simplest explanation and regarded it as the smell of electricity. We now know that an electrical discharge, either as a spark from an induction coil or in the shape of lightning, converts some of the oxygen in the air into another substance, ozone, which is responsible for this peculiar smell. To speak of ozone as another substance is both right and wrong. It is right because in regard to the properties which it possesses, ozone is quite distinct from oxygen. In some respects it behaves like intensified oxygen, oxidizing things which that gas cannot touch. An illustration of this is the extraordinary effect which it has on mercury. The nearest trace of ozone introduced into a vessel containing the metal seems to scare it out of its usual behavior. The bright lustrous surface becomes dull and unresponsive. Instead of moving about freely, it sticks to the glass as if it were greased. In a second sense it is wrong to speak of ozone as another substance than oxygen, for they are simply two forms of the same element, allotropic forms as the chemists call them. The existence of phosphorus and carbon in more than one modification was attributed to a different arrangement of the molecules, but such an explanation could not possibly be correct in the case of gases like oxygen and ozone. For as it has been pointed out already, the molecules of a gas are rushing hither and thither at a high speed, and any definite arrangement of these particles is quite out of the question. The few cases discussed in this chapter of elements existing in more than one form are simply illustrations of what has been observed all over the field of chemistry. It is very frequently found that two compound substances with the same chemical composition are quite distinct in their outward appearance and general behavior. In some instances, the difference is merely one of crystalline form and is to be attributed to a different arrangement of the molecules. In other compounds, however, the origin of the distinction is far more subtle and is found in a different arrangement of the atoms within the molecule. The story of the way in which chemists have discovered the internal structure and anatomy of the molecule is very fascinating, but it is long and intricate, and would detain us from excursions into interesting fields which are close at hand. One other curious phenomenon, however, deserves notice here. When a compound exists in two crystalline modifications, it is very commonly observed that one of the forms has but little persistence and changes into the other on the slightest provocation. Some years ago, the writer came across an interesting case of this kind. He obtained a substance which crystallized in the form of shiny leaflets, but these were no sooner produced than they began to change entirely of their own accord into little needle-shaped crystals. The first form of the substance, in fact, never existed for more than a few minutes. A host of similar interesting cases might be quoted, but enough perhaps has been said to convince the reader that the study of the various forms in which an element or compound can exist reveals nature in very curious moods and brings the chemist into touch with some of the most interesting problems of matter. End of Chapter 5 Chapter 6 of the Romance of Modern Chemistry This LibriVox recording is in the public domain. The Romance of Modern Chemistry by James C. Phillip Chapter 6 Metals Common and Uncommon No one can fail to notice that metals and alloys play a very important part in the economy of modern life. It has not always been so in the history of the world. For, as every schoolboy knows, there was a time when tools and weapons were made exclusively of stone. That primitive stage in man's conquest of nature was followed by the Bronze Age and the Iron Age, and ultimately when Greece and Rome were at the height of their glory as many as seven metals were known and utilized. At the present time the number of known metals is very much greater and innumerable alloys made by mixing two or more metals find application in our technical and social life. We see them everywhere from powerful engines and gigantic bridges down to needles and pins. We carry them about with us on our boots, in our pockets, on our fingers, in our hair, and sometimes even in our mouths. The majority of the 70 known elements are metals and among these there are all sorts and conditions. Only one is a liquid, mercury or quicksilver, and its curious combination of the properties of a metal and those of a liquid render it useful for many special purposes. In the same way, however, as liquid water may be converted into steam by heating and into ice by cooling, so the liquid metal may be boiled producing mercury vapor or it may be solidified by the action of cold. Indeed during winter in extremely cold countries like Siberia the mercury in the bowls of the thermometers may be frozen. This happens when the temperature falls 40 degrees below zero Fahrenheit. Metals exhibit great variety of density and it must not be supposed that a metal is necessarily a heavy substance. It is true that mercury is nearly 14 times as heavy as water and that gold is about 19 times as heavy. Yet there are metals, sodium for instance, which are lighter than water. If a piece of this metal is thrown on water it swims about on the surface. The reader may remember aluminum as a comparatively light metal, the weight of which bulk for bulk is only one seventh that of gold. Only a few of the metals are found in the uncombined or native condition. These are the so-called noble metals, gold, platinum, etc., which are distinguished by the fact that they do not tarnish and are not readily attacked by acids. Other metals occur in the form of ores and have to be extracted from these by laborious processes. However it has come about the majority of the metals have combined with oxygen or sulfur and their ores consist therefore mainly of oxides or sulfides mixed naturally with a smaller or greater amount of earthy matter. The operations or metallurgical processes necessary for winning metals from their ores are modified by the idiosyncrasies of the particular metal which is salt but the essential chemical reaction involved is generally the removal of oxygen from the ore by the agency of carbon. If the ore does not readily consist of the oxide the latter is obtained by roasting the sulfide in a current of air by which process the sulfur in the sulfide is replaced by oxygen. The oxide is mixed with coke which contains a high proportion of carbon and the mixture is heated in a furnace a flux such as lime being added to remove the earthy matter from the ore in a fluid form. At the high temperature of the furnace the carbon in the coke deprives the metal of its oxygen and carries it off in the form of carbon monoxide or carbon dioxide. The metal is thus obtained in the free state and is generally run out of the furnace in a molten condition. While the earthy material that was in the ore is separated along with the flux as slag. Perhaps the commonest example of such a metallurgical operation is iron smelting a process which may be seen at work in many parts of Great Britain. In the case of iron it is desirable to inform ourselves a little more about what is done with the crude metal obtained from the blast furnace and it is well that we should understand the chemical differences between the various kinds of iron which are of technical importance namely cast iron wrought iron and steel. The different properties which characterize these varieties of the metal show in a very interesting manner how the behavior of a pure substance is modified by the presence of small quantities of foreign matter. In a description given in an earlier chapter of the attempts which have been made to manufacture diamonds it was said that the molten iron dissolves carbon. Since now in the process of iron smelting the fused metal has been in contact with coke in the furnace it is not surprising that the crude metal which is taken out of the furnace contains an appreciable amount of carbon as much as 3 to 5 percent. It is run into molds and is then known as cast or pig iron. Careful examination has shown that of the total carbon present in cast iron some has combined with the metal to form a compound known as a carbide while the rest has crystallized out during the cooling in the form of graphite. If the carbon is removed from cast iron as completely as possible we get wrought iron which contains only about one-tenth of one percent of carbon and differs very notably in its properties from cast iron. In the first place wrought iron can be welded that is if two pieces of this material are made red hot they soften and in this state may be hammered together. This cannot be done with cast iron which is a hard brittle crystalline substance. Again cast iron is much more easily melted than wrought iron. The latter is very nearly pure metal whereas the former contains an appreciable quantity of foreign material. Now it is a well known fact that if a small quantity of a foreign body is added to a pure substance the melting point of the mixture is lower than that of the pure substance. Salt water for example contains much more dissolved matter than fresh water and is more difficult to freeze. Or to put it the other way round ice melts at a lower temperature in salt water than it does in fresh. In fact a strong solution of common salt in water will not freeze even at zero degrees Fahrenheit. The fact that cast iron melts more easily or has a lower freezing point than wrought iron is therefore an illustration of a very general principle. The reader will observe that the freezing and melting points are to be regarded as the same temperature and this is always so if we are dealing with a pure substance. The difference is merely this that if we are thinking of the solid being converted into liquid the temperature at which this takes place is called the melting point. If on the other hand we are thinking of the change of liquid into solid the temperature at which this change occurs is called the freezing point. The two temperatures are the same if the substance is pure. The third variety of iron namely steel is intermediate between cast and wrought iron in regard to the amount of carbon which it contains. The remarkable thing about steel is that when it is heated and then suddenly cooled by plunging into cold water it becomes exceedingly hard so much so that it has the power of scratching glass. Curiously enough if this hard steel is again heated and then allowed to cool slowly it is found to be nearly as soft as ordinary iron. By regulating the temperature to which the hardened steel is exposed the second time any required degree of hardness may be attained. Articles made of steel such as razors, scissors and watch springs are therefore first hardened and then tempered by heating them to a point between 430 and 550 degrees Fahrenheit the temperature varying according to the purpose for which the article is to be used. A razor for example is heated only to 430 degrees a temperature at which the metal acquires superficially a pale yellow color due to the formation of a film of oxide. Watch springs or sword blades on the other hand which should be softer and more elastic are tempered by heating to 550 degrees and the color of the surface film passes through various shades yellow, brown, purple and blue as the temperature rises. The degree of heat attained in tempering may in fact be judged from the color of the surface. Thus hardened steel which has been heated to 430 degrees and then allowed to cool slowly is said to be tempered to the yellow and is hard enough to make a fine cutting edge. It must be remembered that steel which has been hardened without being tempered is of no use for ordinary purposes it is too brittle. It is many centuries now since man first began to discover the valuable properties of iron and the passage of time has only led to a gradually widened range of application and to improve methods of production. One can name metals however which for a long time after their discovery were regarded as curiosities and have only recently and more or less suddenly been in large demand as their useful properties have been realized. Aluminum is a notable example of this. Fifty or sixty years ago it cost twenty shillings an ounce. Now it can be purchased for less than a shilling a pound. The very high price of the metal was due not to scarcity of material from which aluminum could be produced but to the fact that there was little or no demand for it and no satisfactory method of extracting it from its ores. As a matter of fact aluminum is one of the most common constituents of the earth's crust occurring in the combined state as mica, feldspar, clay, and slate. It is in many respects a remarkable metal. It is exceedingly light and yet unlike most other metals of this class is not easily tarnished even in the presence of moisture. On account of its lightness it is extensively used in military fittings while its resistance to the action of animal and vegetable juices renders it serviceable in the manufacture of cooking utensils. Although aluminum in the mass is not easily oxidized in air probably because it gets coated with a thin film of oxide which acts as a protective layer the powdered metal burns vigorously like magnesium when it is heated and in this way a very high temperature is produced. If the oxygen which is necessary for the combustion of the aluminum is mixed with the metal at the start instead of coming from the air as the burning proceeds an even higher temperature can be reached. But how, the reader may ask, can we mix a gas with a solid? In the literal sense certainly this cannot be done for to burn half an ounce of aluminum powder as much as 15 pints of oxygen gas would be required but the oxygen may be mixed with the aluminum in a compact or condensed condition in the form of some compound out of which the aluminum has no difficulty in extracting it. Iron oxide is such a compound so if a mixture of powdered aluminum and iron oxide known as thermit is ignited at one point an action sets in which spreads through the whole mass giving out intense heat and resulting in the formation of aluminum oxide and molten metallic iron. The aluminum in fact feeds on the oxygen of which it has deprived the iron. The heat produced in this competition for the oxygen is so intense that if some thermit mixture is placed on an iron plate half an inch thick and ignited a hole is melted in the plate. The heat stored up in thermit may however be turned to more practical account in the following interesting manner. If the ends of two steel rails are pressed together and some of the intensely hot fluid iron produced in the thermit reaction is run out of a crucible onto the junction the crevices are filled. The heat is such that the ends of the rails are softened and may be welded by the applied pressure so that a sound joint is made. In a similar way thermit may be employed for repairing iron shafts and pipes. In aluminum we have an example of a metal the price of which has fallen in a remarkable manner not because fresh sources of the metal have been discovered but because the increased demand has led to cheaper and more efficient methods of production. There are other metals however which are costly not because there is any difficulty about their extraction but because the natural supply is limited. Platinum is a case in point endowed with unique and valuable properties it is comparatively rare and possibly the reader has never seen a specimen. It costs nearly twice as much as gold. Platinum is a silvery metal 21 times as heavy as water bulk for bulk and does not rust or tarnish. Like gold it is a noble metal and is not dissolved by any single acid which we know not even by aquafortis nitric acid. A mixture of nitric and hydrochloric acids however will dissolve both platinum and gold and it was the power of this mixture to attack the latter metal which led the alchemist to speak of it as aqua regia. Platinum is in a special sense the chemist's metal it's very high melting point 3200 degrees Fahrenheit and its chemical inertness make it valuable to him and platinum crucibles are to be reckoned among the indispensables of a properly equipped chemical laboratory. There is another characteristic of platinum which to the casual reader may seem most insignificant but which as it turns out is of the greatest importance in a certain manufacturer. This is the fact that with the rising temperature platinum expands at nearly the same rate as glass. Why should that be of any consequence the reader may ask and what has platinum got to do with glass? Well it is generally recognized that bodies expand when heated they become more bulky as the temperature rises. As a rule the expansion differs with different substances but it so happens that platinum expands to the same extent as glass for a given rise of temperature. We may realize the significance of this fact when we remember that it is sometimes necessary to pass a metal wire through the walls of a glass vessel. This has to be done for example in the electric glow lamp the illuminating power of which is due to a carbon filament raised to a white heat by the passage of an electric current. As the carbon would soon burn away if it were surrounded by air the little glass globe which protects the filament must be freed from air and then sealed up. The wires therefore which carry the current must pass through the walls of the globe and the questionnet once arises what metal should be used for these wires? Copper, the metal which is commonly used for electric wiring would be quite unsuitable because it does not expand and contract with change of temperature at the same rate as glass. If we passed a copper wire through a piece of glass while the ladder is hot and soft and melted the glass all around the wire then on cooling owing to the unequal contraction of the metal in the glass a condition of strain would be produced leading finally to the fracture of the glass. A metal is required which expands and contracts at the same rate as glass and the one metal with this characteristic is platinum. Hence it comes that an electric glow lamp has two little pieces of platinum connecting the ends of the filament inside with the terminals outside. Thus it is that what may seem at first to be nothing more than a dry laboratory fact without any practical bearing may turn out to be of the greatest importance for the requirements of everyday life. Such a case of the discovery of a new use for a metal and a consequent fresh demand for it might be paralleled by what has happened recently in connection with tantalum. This is a rare metal and up to within a year or two ago very little attention had been paid to it as a glance at the chemical textbook will show. It has been discovered however that tantalum has certain properties of commercial value and people are now on the lookout for fresh sources of this somewhat scarce material. The illuminating power of the electric glow lamp depends, as has been said already, on a carbon filament being raised to an incandescence by an electric current. Now these carbon filaments are very fragile creations and one might at first be inclined to wonder why fine metallic wires are not used instead for it is well known that a metallic wire is similarly heated by the passage of a current. The explanation is simple. In order to get a respectable light from an incandescent metal wire we should have to raise it to a temperature at which it would melt. This would happen even with platinum for the temperature of the carbon filament in an electric glow lamp is several hundred degrees higher than the melting point of that metal. It is just here that the valuable properties of tantalum come in. Briefly stated they are these. Tantalum can be drawn into very fine wire about one thousandth of an inch in diameter and its melting point is exceedingly high so high that the fine wire may be raised to a white heat by an electric current without melting. The tantalum lamp then which is now on the market is exactly analogous to the electric glow lamp except that the filament is made of tantalum instead of carbon. As regards efficiency the tantalum lamp compares favorably with the ordinary glow lamp and it is said to have a longer life. So the time may come if sufficient tantalum can be procured that the carbon filament will have become merely a curiosity. Tungsten and osmium are other out-of-the-way metals which have recently found an important application in the manufacture of electric lamps so that tantalum is not the only competitor in the field against the carbon filament. The application of electricity to all sorts of objects has led to extended demands for other metals than those just quoted. Copper for instance which offers very slight resistance to the passage of a current is much in demand for electric wiring. Enormous quantities of the metal are now devoted to this purpose. Copper enters also into the composition of many alloys brass, bronze, and the like but the subject of alloys is a big one and may be reserved for another chapter. End of Chapter 6 Chapter 7 of the Romance of Modern Chemistry This LibriVox recording is in the public domain. The Romance of Modern Chemistry by James C. Phillip Chapter 7 where two metals are better than one In the foregoing chapter we have had illustrations of the differences exhibited by metals in regard to specific gravity and fusibility but there are of course many other properties hardness, malleability, ductility, and the like which have to be taken into account when we are considering the suitability of a given metal for a certain purpose. Frequently it happens that the metal is suitable for the purpose in all respects save one in which case it may be possible to correct the deficiency by adding another metal provided at the same time that this second constituent does not detract from the valuable properties of the first. The effect of the presence of carbon and iron might be considered as an illustration of this principle but carbon is not a metal and the amount present is not great so that wrought iron, cast iron, and steel are rather in a class by themselves. We may make good the deficiencies of a metal by the addition of another in more than one way. It is not always necessary actually to mix the metals. We may put one on top of the other as in tin plate or galvanized iron. In regard to strength, durability, and cheapness iron is an excellent material but the weak feature about it is its liability to corrode when exposed to moist atmosphere. It rusts. Articles made of iron which have to be exposed to air and moisture must therefore be protected. In the case of large structures such as bridges, locomotives, and steamers this is done by painting them but with smaller and more easily handled articles the same end is attained by coating them with a layer of another metal which is not easily corroded by the action of moist air. Tin and zinc are metals which fulfill these conditions and they are further comparatively fusible so that a sheet of iron may be easily coated with either by simply dipping it into a bath of the fused metal. Iron coated in this way with a layer of tin is known as tin plate. Similarly treated with zinc it is known as galvanized iron. So that things are not always what they seem. Even the common pin is a fraud in this sense for if we could open it out we should find brass wire in the inside quite different from the white metal on the outside. Brass as the reader probably knows is a yellow alloy containing a lot of copper and the easiest way of showing that pins contain this metal is to dissolve one in nitric acid. The pin is gradually consumed by the acid it ultimately disappears and a blue liquid remains similar to what it's obtained by treating a piece of pure copper in the same way. So we may conclude that there is copper in the pin. The white outside is a coating of tin. This however is not put on as in tin plate by dipping in a bath of the fused metal but by another interesting method. The reader may recollect that among the things that lent support to the alchemist's belief in the transmutation of metals was the observation that a piece of iron immersed in a solution containing copper acquires the appearance of copper. This little trick can be performed with other metals also and is applied in the manufacture of pins. The brass wires which form the substance of the pins are put in a solution containing tin and the result is that a coating of this metal is deposited on the brass. Such a method of depositing one metal on another is very closely related to the process of electroplating. When an electric current is passed between two metal rods or electrodes immersed in a salt solution the salt is decomposed and the metallic constituent is deposited on one of the electrodes. Suppose now that the solution contains a salt of silver and that we replace the rod on which the metallic constituent is deposited by a spoon made of some alloy. Then on passing the current the usual thing happens and we get a fine coherent deposit of silver on the spoon. The latter is electroplated. In this way articles made of common metals or alloys may be plated with gold, silver, copper, or nickel. For example, we can protect steel articles such as bicycle fittings from atmospheric corrosion by plating them with nickel. Articles on the other hand which are to be used for the table or for ornament may be similarly coated with silver. Spoons and forks for instance are generally made of Britannia metal an alloy containing mostly tin and antimony or of German silver an alloy of copper, zinc, and nickel. When these articles are coated with silver they are less easily attacked by acid liquids and at the same time their appearance is improved. Plating after all is a device for hiding the shortcomings of one metal by covering it with another. There is however a second way of making good the deficiencies of a metal and that is by mixing it thoroughly with another of different character. In what way the properties of one metal are modified by thus alloying it with a second may be best understood from a few examples. The ubiquitous penny piece is commonly known as a copper. This metal in the pure condition however would be too soft for the wear and tear which a coin has to undergo and consequently five percent of foreign metal mostly tin is added to the pure copper in order to harden it. The curious thing is that tin itself is quite a soft metal and yet the addition of five percent of it to pure copper produces an alloy which is much harder than copper itself. Similarly pure gold and silver are too soft to be used either for coins or ornaments and for this purpose they must be hardened by the addition of another metal generally copper. The color of an English silver coin does not betray the presence of copper but if a three penny bit were dissolved in nitric acid we should get the blue solution which is characteristic of copper. Since pure silver dissolved in nitric acid gives a colorless solution we may conclude that our silver coins contain copper as a matter of fact they have seven and a half percent of that metal. An English gold coin is also alloyed with copper to the extent of one part for every 11 parts of gold. Such an alloy is described as 22 karat gold because out of 24 parts of the alloy 22 parts are pure gold. For ornaments other alloys are made containing less of the precious metal and described as 18, 12 or 9 karat gold. These figures indicating that 24 parts of the alloy contain 18, 12 or 9 parts of pure gold respectively. Another property which is altered by adding a second metal is the fusibility and an alloy as a rule melts at a much lower temperature than one would expect from the melting points of the constituent metals. The influence of a foreign body on the melting point of a pure substance has already been referred to in connection with cast and wrought iron but with alloys the effects are much more striking. Common solder is a case in point. Soldering consists in joining two metals by an alloy which is more easily melted than either and which at the same time will coalesce with each metal. Tin and lead a mixture of which forms ordinary plumber solder melt at 440 degrees and 617 degrees Fahrenheit respectively while the solder itself melts at 374 degrees. The reader may already have remarked the frequency with which tin is used either to plate or alloy with other metals and it is in fact very seldom employed by itself. Even the so-called tin foil in which chocolates are wrapped contains lead and the utensils which we call tins are generally iron plated with tin. In the case of solder as we have seen the alloy melts at a temperature lower than the melting point of either constituent but of the lowering of melting point produced by mixing far more striking examples are obtained when we take four metals to make an alloy. Lead, tin, bismuth and cadmium melt at 617 degrees 440 degrees 514 degrees and 608 degrees respectively and yet by mixing these metals in certain proportions we can prepare an alloy which melts in hot water and is known as fusible metal. These easily melted alloys are put to some curious uses for example in connection with fire alarms. A quantity of fusible metal is arranged in a receptacle in such a way that when a certain temperature has been exceeded the alloy melts and releases a spring or allows a lever to fall. By this device an electric current is closed and a bell is rung. Then again fusible alloys play a useful part in the sprinklers which are fitted up in factories and workshops. These consist of water pipes lead round the upper part of a room and at intervals on the pipes there are valves secured by fusible alloy. If a fire should break out in any room fitted with such a sprinkler the heat will melt the fusible alloy at one or more of the valves. The water will burst out and there is a fair chance that the fire will be extinguished before it has attained very large proportions. With such examples before us of the way in which the hardness and fusibility of a metal are altered by the addition of another we are led up to the question are these alterations due to the formation of a new substance a compound of the metals or are they accounted for by the mere mixture of the constituents? The difference between a mechanical mixture of two elements and the compound formed by their chemical combination has already been discussed. We must next try to decide whether the features which we generally notice in chemical action are to be observed when we make two metals into an alloy. Now it must be admitted that it is rather difficult to settle the question whether an alloy is a mixture or a compound. In some respects it is true that the mixing of two metals resembles what takes place when two elements combine. Thus we have seen that when iron and sulfur act on each other an enormous amount of heat is liberated and such liberation of heat very frequently accompanies chemical action. Well a similar thing occurs when we add sodium to mercury to make an alloy or an amalgam as it is called when mercury is one constituent. Each addition of sodium is accompanied by a flash of light. So also when a piece of aluminum is added to fuse gold an extraordinary evolution of heat is observed and the molten mass is raised to incandescence. Such an occurrence may be taken as evidence that chemical combination has taken place. Again the color of some alloys is markedly different from that of the constituents. Silver and zinc are both white metals and yet they form a beautiful pink alloy. Gold and aluminum also furnish us with an illustration for when they are mixed in certain proportions a brilliant purple alloy is obtained quite distinct in color from both constituents. In spite of these examples however it must be said that we do not observe in the formation of alloys generally such a thorough going change of properties as commonly results from chemical combination. The question therefore of mechanical mixture versus chemical compound is not so easily decided. Modern investigators have tackled the problem by studying the freezing points of alloys in their relation to the freezing points of the constituent metals. And in the course of these investigations many interesting results have been brought to light. It is almost the invariable rule that when a little of a metal B is added to a metal A the fused alloy begins to solidify at a lower temperature than pure A. It is said to have a lower freezing point. As we go on adding more and more of the metal B the alloys produced have lower and lower freezing points. The same series of phenomena is observed when we add increasing quantities of the metal A to the pure metal B. Suppose we try to picture these results graphically that is with the help of a curve. In doing this we represent the composition of the alloy by distances measured along the horizontal line while the temperature at which the alloys freeze are represented by lengths measured vertically. This is a very common method of summarizing the results of scientific investigation and of showing the way in which one quantity depends on or varies with another. An example of such a use of curves is furnished by the card which comes off an aneroid barometer at the end of a week. On this card vertical distances represent the height of the barometer and horizontal distances represent intervals of time. The curve traced on the card shows the way in which the height of the barometer has varied during the past week. If now we similarly represent the way in which the freezing point of an alloy varies with its composition we should obtain in many cases at least a curve in two branches as shown in figure 1A. Careful investigation has shown that these are the cases in which chemical combination has not taken place. The solid which separates out when the fused alloy begins to solidify is either pure A or pure B never a compound of the two metals. In the cases where the two metals do form a compound the curve showing the variation of freezing point with composition is of a different character. There is then an intermediate branch of the freezing point curve shaped more or less like a camels hump see figure 1B. The temperature corresponding to the summit of the hump is the freezing point of the compound which is formed and the composition of the alloy which has this maximum freezing point gives the composition of the compound. It is interesting now to find that the existence of compounds of mercury and sodium and of gold and aluminum which we suspected from their behavior on mixing is confirmed by the study of the freezing point curves for alloys of these metals. In the case of mercury and sodium the freezing point curve has a hump the top of which is far above the freezing point of either constituent so that the existence of a compound is here proved very definitely. The freezing point curve for alloys of gold and aluminum has actually two separate humps showing that these metals combine to form two compounds with different proportions of the constituents. One of the humps corresponds to the formation of the beautiful purple alloy already referred to and it is very remarkable that this compound containing 20% of aluminum should melt at the same temperature as pure gold. Another method of investigating the nature of alloys which has recently been employed with success is the study of their structure with the aid of the microscope. If a section of an alloy is polished treated with a little acid or other corrosive liquid so as to bring out the details on the surface and put under the microscope one can observe an outline of crystalline structure which cannot be detected by the unaided eye. Those who have experience in this sort of investigation can actually distinguish the various kinds of crystals which have separated while the alloy was passing from the fused to the solid condition. End of chapter 7