 CHAPTER XII The Experimental Study of Combustion made by Lavoisier proved the correctness of that part of Stahl's logistic theory, which asserted that all processes of combustion are very similar, but also proved that this likeness consists in the combination of a distinct gaseous substance with the material undergoing combustion and not in the escape, therefore, of the principle of fire as asserted by the theory of Stahl. After about the year 1790 it was necessary to think of combustions in the air as combinations of a particular gas or air with the burning substances or some portions of them. This description of processes of burning necessarily led to a comparison of the gaseous constituent of the atmosphere which played so important a part in these processes with the substances which were burned. It led to the examination of the compositions of many substances and made it necessary to devise a language whereby these compositions could be stated clearly and consistently. We have seen in former chapters the extreme haziness of the alchemical views of composition and the connections between composition and properties, although Boyle had stated very lucidly what he meant by the composition of a definite substance, about a century before Lavoisier's work on combustion, nevertheless the views of chemists concerning composition remained very vague and incapable of definite expression until the experimental investigations of Lavoisier enabled him to form a clear mental picture of chemical changes as interactions between definite quantities of distinct substances. Boyle said in 1689, I mean by elements certain primitive and simple or perfectly unmixed bodies which not being made of any other bodies or of one another are the ingredients of which all those called perfectly mixed bodies are immediately compounded and into which they are ultimately resolved. End footnote. Let us consider some of the work of Lavoisier in this direction. I select his experimental examination of the interactions of metals and acids. Many experimenters had noticed that gases or airs as they were called up till near the end of the 18th century are generally produced when metals are dissolving in acids. Most of those who noticed this said that the gases came from the dissolving metals. Lavoisier said they were produced by the decomposition of the acids. In order to study the interaction of nitric acid and mercury, Lavoisier caused a weighed quantity of the metal to react with a weighed quantity of the acid and collected the gas which was produced. When all the metal had dissolved he evaporated the liquid until a white solid was obtained. He heated this solid until it was changed to the red substance called at that time red precipitate and collected the gas produced. Finally Lavoisier strongly heated the red precipitate. It changed to a gas which he collected and mercury which he weighed. The weight of the mercury obtained by Lavoisier at the end of this series of changes was the same, less a few grains, as the weight of the mercury which he had caused to react with the nitric acid. The gas obtained during the solution of the metal in the acid and during the decomposition of the white solid by heat was the same as a gas which had been prepared by priestly and called by him nitrous air, and the gas obtained by heating the red precipitate was found to be oxygen. Lavoisier then mixed measured volumes of oxygen and nitrous air, standing over water. A red gas was formed and dissolved in the water, and Lavoisier proved that the water now contained nitric acid. The conclusions regarding the composition of nitric acid drawn by Lavoisier from these experiments was that nitric acid is nothing else than nitrous air combined with almost its own volume of the purest part of atmospheric air and a considerable quantity of water. Lavoisier supposed that the stages in the complete reaction between mercury and nitric acid were these, the withdrawal of oxygen from the acid by the mercury, and the union of the compound of mercury and oxygen thus formed with the constituents of the acid which remained when part of its oxygen was taken away. The removal of oxygen from nitric acid by the mercury produced nitrous air when the product of the union of the oxide of mercury and the nitric acid deprived of part of its oxygen was heated, more nitrous air was given off, and oxide of mercury remained, and was decomposed at a higher temperature into mercury and oxygen. Lavoisier thought of these reactions as the tearing asunder by mercury of nitric acid into definite quantities of its three components, themselves distinct substances, nitrous air, water and oxygen, and the combination of the mercury with a certain measurable quantity of one of these components, namely oxygen, followed by the union of this compound of mercury and oxygen with what remained of the components of nitric acid. Lavoisier had formed a clear, consistent and suggestive mental picture of chemical changes. He thought of a chemical reaction as always the same under the same conditions, as an action between a fixed and measurable quantity of one substance, having definite and definable properties with fixed and measurable quantities of other substances, the properties of each of which were definite and definable. Lavoisier also recognised that certain definite substances could be divided into things simpler than themselves, but that other substances refused to undergo simplification by division into two or more unlike portions. He spoke of the object of chemistry as follows. In submitting to experiments the different substances found in nature, chemistry seeks to decompose these substances and to get them into such conditions that their various components may be examined separately. Chemistry advances to its end by dividing, subdividing and again subdividing, and we do not know what will be the limits of such operations. We cannot be certain that what we regard as simple today is indeed simple. All we can say is that such a substance is the actual term we are at chemical analysis has arrived, and that with our present knowledge we cannot subdivide it. Footnote I have given a free rendering of Lavoisier's words. End footnote. In these words Lavoisier defines the chemical conception of elements. Since his time an element is the actual term we are at chemical analysis has arrived. It is that which with our present knowledge we cannot subdivide, and as a working hypothesis the notion of element has no wider meaning than this. I have already quoted Boyle's statement that by elements he meant certain primitive and simple bodies not made of any other bodies or of one another. Boyle was still slightly restrained by the alchemical atmosphere around him. She was still inclined to say, this must be the way nature works. She must begin with certain substances which are absolutely simple. Lavoisier had thrown off all the trammels which hindered the alchemists from making rigorous experimental investigations. If one may judge from his writings he had not struggled to defree himself from these trammels. He had not slowly emerged from the quagmires of alchemy and painfully gained firmer ground. The extraordinary clearness and directness of his mental vision had led him straight to the very heart of the problems of chemistry, and enabled him not only calmly to ignore all the machinery of elements, principles, essences, and the like which the alchemists had constructed so laboriously, but also to construct in place of that mechanism which hindered inquiry genuine scientific hypotheses which directed inquiry and were themselves altered by the result of the experiments they had suggested. Lavoisier made these great advances by applying himself to the minute and exhaustive examination of a few cases of chemical change and endeavouring to account for everything which took part in the processes he studied by weighing or measuring each distinct substance which was present when the change began and each which was present when the change was finished. He did not make haphazard experiments. He had a method, a system, he used hypotheses, and he used them rightly. Systems in physics, Lavoisier writes, are but the proper instruments for helping the feebleness of our senses. Properly speaking they are methods of approximation which put us on the track of solving problems. They are the hypotheses which successively modified, corrected, and changed by experience, ought to conduct us some day by the method of exclusions and eliminations to the knowledge of the true laws of nature. In a memoir wherein he is considering the production of carbonic acid and alcohol by the fermentation of fruit juice, Lavoisier says, it is evident that we must know the nature and composition of the substances which can be fermented and the products of fermentation, for nothing is created either in the operations of art or in those of nature, and it may be laid down that the quantity of material present at the beginning of every operation is the same as the quantity present at the end, that the quality and quantity of the principles are the same, and that nothing happens save certain changes, certain modifications. On this principle is based the whole art of experimenting in chemistry. In all chemical experiments we must suppose that there is a true equality between the principles of the substances which are examined and those which are obtained from them by analysis. Footnote. Lavoisier uses the word principle here and elsewhere to mean a definite homogeneous substance. He uses it as synonymous with the more modern terms element and compound. End footnote. If Lavoisier's memoirs are examined closely, it is seen that at the very beginning of his chemical inquiries he assumed the accuracy and the universal application of the generalization, nothing is created either in the operations of art or in those of nature. Naturalists had been feeling their way for centuries towards such a generalization as this. It had been in the air for many generations. Sometimes it was almost realized by this or that investigator, then it escaped for long periods. Lavoisier seems to have realized, by what we call intuition, that however great and astonishing may be the changes in the properties of the substances which mutually react, there is no change in the total quantity of material. Not only did Lavoisier realize and act on this principle, he also measured quantities of substances by the one practical method, namely by weighing. And by doing this he showed chemists the only road along which they could advance towards a genuine knowledge of material changes. The generalization expressed by Lavoisier in the words I have quoted is now known as the law of the conservation of mass. It is generally stated in some such form as this. The sum of the masses of all the homogeneous substances which take part in a chemical or physical change does not itself change. The science of chemistry rests on this law. Every quantitative analysis assumes the accuracy and is a proof of the validity of it. I have considered the law of the conservation of mass in some detail in chapter 4 of the story of the chemical elements in footnote. By accepting the accuracy of this generalization and using it in every experiment, Lavoisier was able to form a clear mental picture of a chemical change as the separation and combination of homogeneous substances. For by using the balance he was able to follow each substance through the maze of changes to determine when it united with other substances and when it separated into substances simpler than itself. CHAPTER XIII THE CHEMICAL ELEMENTS CONTRASTED WITH THE ALCHEMICAL PRINCIPLES It was known to many observers in the later years of the 17th century that the product of the calcination of a metal weighs more than the metal, but it was still possible at that time to assert that this fact is of no importance to one who is seeking to give an accurate description of the process of calcination. Weight, which measures mass or quantity of substance, was thought of in these days as a property like color, taste or smell, a property which was sometimes decreased and sometimes increased by adding one substance to another. Students of natural occurrences were, however, feeling their way towards the recognition of some property of substances which did not change in the haphazard way wherein most properties seemed to alter. Lavoisier reached this property at one bound. By his experimental investigations he taught that, however greatly the properties of one substance may be masked or altered by adding another substance to it, yet the property we call mass and measure by weight is not affected by these changes. Lavoisier showed that the mass of the product of the union of two substances is always exactly the sum of the masses of these two substances, and the sum of the masses of the substances wherein two one substance is divided is always exactly equal to that mass of the substance which is divided. For the undefined, ever-changing, protean essence or soul of a thing, which the alchemists thought of as hidden by wrappings of properties, the exact investigations of Lavoisier and those of others who worked on the same lines as he substituted this definite, fixed, unmodifiable property of mass. Lavoisier and those who followed in his footsteps also did away with the alchemical notion of the existence of an essential substratum independent of changes in those properties of a substance which can be observed by the senses. For the experimental researches of these men, obliged naturalists to recognize that a change in the properties of a definite homogeneous substance such as pure water, pure chalk or pure sulphur is accompanied, or as we generally say is caused, by the formation of a new substance or substances, and this formation, this apparent creation of new material is affected either by the addition of something to the original substance or by the separation of it into portions which are unlike it and unlike one another. If the change is a combination or coalescence of two things into one, then the mass and hence the weight of the product is equal to the sum of those masses and hence those weights of the things which have united to form it. If the change is a separation of one distinct substance into several substances, then the sum of the masses and hence the weights of the products is equal to that mass and hence that weight of the substance which has been separated. Consider the word water and the substance represented by this word. In chapter four I gave illustrations of the different meanings which have been given to this word. It is sometimes used to represent a material substance, sometimes a quality more or less characteristic of that substance, and sometimes a process to which that substance and many others like it may be subjected. But when the word water is used with a definite and exact meaning, it is a succinct expression for a certain group or co-location of measurable properties which are always found together and is therefore thought of as a distinct substance. This substance can be separated into two other substances very unlike it and can be formed by causing these to unite. 100 parts by weight of pure water are always formed by the union of 11.11 parts of hydrogen and 88.89 parts of oxygen and can be separated into these quantities of those substances. When water is formed by the union of hydrogen and oxygen in the ratio of 11.11 parts by weight of the former to 88.89 of the latter, the properties of the two substances which coalesce to form it disappear, except their masses. It is customary to say that water contains hydrogen and oxygen, but this expression is scarcely an accurate description of the facts. What we call substances are known to us only by their properties, that is the ways wherein they act on our senses. Hydrogen has certain definite properties. Oxygen has other definite properties, and the properties of water are perfectly distinct from those of either of the substances which it is said to contain. It is therefore somewhat misleading to say that water contains substances the properties whereof except their masses disappeared at the moment when they united and water was produced. Nevertheless, we are forced to think of water as, in a sense, containing hydrogen and oxygen. For one of the properties of hydrogen is its power to coalesce or combine with oxygen to form water, and one of the properties of oxygen is its ability to unite with hydrogen to form water, and these properties of those substances cannot be recognized or even suspected unless certain definite quantities of the two substances are brought together under certain definite conditions. The properties which characterize hydrogen and those which characterize oxygen, when these things are separated from all other substances, can be determined and measured in terms of the similar properties of some other substance taken as a standard. These two distinct substances disappear when they are brought into contact under the proper conditions, and something, water, is obtained whose properties are very unlike those of hydrogen or oxygen. This new thing can be caused to disappear, and hydrogen and oxygen are again produced. This cycle of changes can be repeated as often as we please. The quantities of hydrogen and oxygen which are obtained when we choose to stop the process are exactly the same as the quantities of those substances which disappeared in the first operation whereby water was produced. Hence water is an intimate union of hydrogen and oxygen, and in this sense water may be said to contain hydrogen and oxygen. The alchemist would have said the properties of hydrogen and oxygen are destroyed when these things unite to form water, but the essence or substratum of each remains. The chemist says you cannot discover all the properties of hydrogen and oxygen by examining these substances apart from one another, for one of the most important properties of either is manifested only when the two mutually react. The formation of water is not the destruction of the properties of hydrogen and oxygen and the revelation of their essential substratum, it is rather the manifestation of a property of each which cannot be discovered except by causing the union of both. There was then a certain degree of accuracy in the alchemical description of the processes we now call chemical changes as being the removal of the outer properties of the things which react and the manifestation of their essential substance, but there is a vast difference between this description and the chemical presentment of these processes as reactions between definite and measurable quantities of elements or compounds or both resulting in the redistribution of the elements or the separation of the compounds into their elements and the formation of new compounds by the recombination of these elements. Let us contrast the two descriptions somewhat more fully. The alchemist wished to affect the transmutation of one substance into another. He despaired of the possibility of separating the elements whereof the substance might be formed, but he thought he could manipulate what he called the virtues of the elements by a judicious use of some more all of the three principles which he named sulphur, salt and mercury. He could not state in definite language what he meant by these principles. They were states, conditions or qualities of classes of substances which could not be defined. The directions the alchemist was able to give to those who sought to affect the changes of one thing into another were these. Firstly, to remove those properties which characterised the thing to be changed and leave only the properties which it shared with other things like it. Secondly, to destroy the properties which the thing to be changed possessed in common with certain other things. Thirdly, to co-mingle the essence of the thing with the essence of something else in due proportion and under proper conditions. And finally, to hope for the best, keep a clear head and maintain a sense of virtue. If he who was about to attempt the transmutation inquired how he was to destroy the specific properties and the class properties of the thing he proposed to change and by what methods he was to obtain its essence and cause that essence to produce the new thing he would be told to travel along the road which was followed by the great architect of the universe in the creation of the world. And if he demanded more detailed directions, he would be informed that the substance where with his experiment began must first be mortified, then dissolved, then conjoined, then putrified, then congealed, then sebated, then sublimed, and fermented, and finally exalted. He would, moreover, be warned that in all these operations he must use not things which he could touch, handle, and weigh, but the virtues, the lives, the souls of such things. When the student of chemistry desires to affect the transformation of one definite substance into another, he is told to determine by quantitative experiments what are the elements and what the quantities of these elements which compose the compound which he proposes to change and the compound into which he proposes to change it, and he is given working definitions of the words element and compound. If the compound he desires to produce is found to be composed of elements different from those which form the compound where with his operation begin, he is directed to bring about a reaction or a series of reactions between the compound which is to be changed and some other co-location of elements, the composition of which is known to be such that it can supply the new elements which are needed for the production of the new compound. Since Lavoisier realized for himself and those who were to come after him the meaning of the terms element and compound, we may say that chemists have been able to form a mental picture of the change from one definite substance to another which is clear, suggestive, and consistent because it is an approximately accurate description of the facts discovered by careful and penetrative investigations. This presentment of the change has been substituted for the alchemical conception which was an attempt to express what introspection and reasoning on the results of superficial investigations guided by specious analogies suggested ought to be the facts. Lavoisier was the man who made possible the more accurate and more far reaching description of the changes which result in the production of substances very unlike those which are changed, and he did this by experimentally analyzing the conceptions of the element and the compound, giving definite and workable meanings to these conceptions, and establishing on an experimental foundation the generalization that the sum of the quantities of the substances which take part in any change is itself unchanged. A chemical element was thought of by Lavoisier the actual term we're at analysis has arrived, a definite substance which we cannot subdivide with our present knowledge, but not necessarily a substance which will never be divided. A compound was thought of by him as a definite substance which is always produced by the union of the same quantities of the same elements and can be separated into the same quantities of the same elements. These conceptions were amplified and made more full of meaning by the work of many who came after Lavoisier, notably by John Dalton, who was born in 1766 and died in 1844. In chapter one I gave a sketch of the atomic theory of the Greek thinkers. The founder of that theory, who flourished about 500 BC, said that every substance is a co-location of a vast number of minute particles which are unchangeable, indestructible, and impenetrable, and are therefore properly called atoms. That the differences which are observed between the qualities of things are due to differences in the numbers, sizes, shapes, positions, and movements of atoms, and that the process which occurs when one substance is apparently destroyed and another is produced in its place is nothing more than a rearrangement of atoms. The supposition that changes in the properties of substances are connected with changes in the numbers, movements, and arrangements of different kinds of minute particles was used in a general way by many naturalists of the 17th and 18th centuries, but Dalton was the first to show that the data obtained by the analyses of compounds make it possible to determine the relative weights of the atoms of the elements. Dalton used the word atom to denote the smallest particle of an element or a compound which exhibits the properties characteristic of that element or compound. He supposed that the atoms of an element are never divided in any of the reactions of that element, but the atoms of a compound are often separated into the atoms of the elements whereof the compound is composed. Apparently without knowing that the supposition had been made more than 2,000 years before his time, Dalton was led by his study of the composition and properties of the atmosphere to assume that the atoms of different substances, whether elements or compounds, are of different sizes and have different weights. He assumed that when two elements unite to form only one compound, the atom of that compound has the simplest possible composition, is formed by the union of a single atom of each element. Dalton knew only one compound of hydrogen and nitrogen, namely ammonia. Analyses of this compound show that it is composed of one part by weight of hydrogen and 4.66 parts by weight of nitrogen. Dalton said one atom of hydrogen combines with one atom of nitrogen to form an atom of ammonia, hence an atom of nitrogen is 4.66 times heavier than an atom of hydrogen. In other words, if the atomic weight of hydrogen is taken as unity, the atomic weight of nitrogen is expressed by the number 4.66. Dalton referred the atomic weights of the elements to the atomic weight of hydrogen as unity, because hydrogen is lighter than any other substance, hence the numbers which tell how much heavier the atoms of the elements are than an atom of hydrogen are always greater than one, are always positive numbers. When two elements unite in different proportions by weight to form more than one compound, Dalton supposed that, in most cases at any rate, one of the compounds is formed by the union of a single atom of each element, the next compound is formed by the union of one atom of the element which is present in smaller quantity, with two, three or more atoms of the other element, and the next compound is formed by the union of one atom of the first element with a larger number, always necessarily a whole number of atoms of the other element than is contained in the second compound, and the next compound is formed by the union of one atom of the first element with a larger number, always necessarily a whole number of atoms of the other element than is contained in the second compound, and so on. From this assumption and the Daltonian conception of the atom, It follows that the quantities by weight of one element which are found to unite with one and the same weight of another element must always be expressible as whole multiples of one number. For if two elements a and b form a compound, that compound is formed by supposition of one atom of a and one atom of b. If more of b is added, at least one atom of b must be added. However much of b is added, the quantity must be a whole number of atoms, and as every atom of b is the same in all respects as every other atom of b, the weights of b added to a constant weight of a must be whole multiples of the atomic weight of b. The facts which were available in Dalton's time confirmed this deduction from the atomic theory within the limits of experimental errors, and the facts which have been established since Dalton's time are completely in keeping with the deduction. Take for instance three compounds of the elements nitrogen and oxygen. That one of the three which contains least oxygen is composed of 63.64% of nitrogen and 36.36% of oxygen. If the atomic weight of nitrogen is taken to be 4.66, which is the weight of nitrogen that combines with one part by weight of hydrogen, then the weight of oxygen combined with 4.66 of nitrogen is 2.66. Note the ratio of 63.64 to 36.36 is equal to 4.66 to 2.66. End note. The weights of oxygen which combine with 4.66 parts by weight of nitrogen to form the second and third compounds respectively must be whole multiples of 2.66. These weights are 5.32 and 10.64. Now 5.32 equals 2.66 times 2, and 10.64 equals 2.66 times 4. Hence the quantities by weight of oxygen which combine with one and the same weight of nitrogen are such that two of these quantities are whole multiples of the third quantity. Dalton's application of the Greek atomic theory to the facts established by the analyses of compounds enabled him to attach to each element a number which he called the atomic weight of the element, and to summarize all the facts concerning the composition of compounds in the statement that the elements combine in the ratios of their atomic weights or in the ratios of whole multiples of their atomic weights. All the investigations which have been made into the compositions of compounds since Dalton's time have confirmed the generalization which followed from Dalton's application of the atomic theory. Even if the theory of atoms were abandoned, the generalization would remain as an accurate and exact statement of facts which hold good in every chemical change that a number can be attached to each element and the weights of the elements which combine are in the ratios of these numbers or whole multiples of these numbers. Since chemists realized the meaning of Dalton's book published in 1808 and entitled a new system of chemical philosophy, elements have been regarded as distinct and definite substances which have not been divided into parts different from themselves and unite with each other in definite quantities by weight which can be accurately expressed as whole multiples of certain fixed quantities, and compounds have been regarded as distinct and definite substances which are formed by the union of and can be separated into quantities of various elements which are expressible by certain fixed numbers or whole multiples thereof. These descriptions of elements and compounds are expressions of actual facts. They enable chemists to state the compositions of all the compounds which are or can be formed by the union of any elements. For example let A, B, C and D represent four elements and also certain definite weights of these elements. Then the compositions of all the compounds which can be formed by the union of these elements are expressed by the scheme A, N, B, M, C, P, D, Q where M, N, P and Q are whole numbers. These descriptions of elements and compounds also enable chemists to form a clear picture to themselves of any chemical change. They think of a chemical change as being one, a union of those weights of two or more elements which are expressed by the numbers attached to these elements, or by whole multiples of these numbers, or two, a union of such weights of two or more compounds as can be expressed by certain numbers or by whole multiples of these numbers, or three, a reaction between elements and compounds or between compounds and compounds resulting in the redistribution of the elements concerned in such a way that the complete change of composition can be expressed by using the numbers or whole multiples of the numbers attached to the elements. How different is this conception of a change wherein substances are formed entirely unlike those things which react to form them from the alchemical presentment of such a process? The alchemist spoke of stripping off the outer properties of the thing to be changed and by operating spiritually on the soul which was thus laid bare, inducing the essential virtue of the substance to exhibit its powers of transmutation. But he was unable to give definite meanings to the expressions which he used. He was unable to think clearly about the transformations which he tried to accomplish. The chemist discards the machinery of virtues, souls and powers. It is true that he substitutes a machinery of minute particles, but this machinery is merely a means of thinking clearly and consistently about the changes which he studies. The alchemist thought vaguely of substance as something underlying and independent of properties. The chemist uses the expression this or that substance as a convenient way of presenting and reasoning about certain groups of properties. It seems to me that if we think of matter as something more than properties recognized by the senses, we are going back on the road which leads to the confusion of the alchemical times. The alchemists expressed their conceptions in what seems to us a crude, inconsistent and very undescriptive language. Chemists use a language which is certainly symbolical but also intelligible and on the whole fairly descriptive of the facts. A name is given to each elementary substance, that is each substance which has not been decomposed. The name generally expresses some characteristic property of the substance or tells something about its origin or the place of its discovery. The names of compounds are formed by putting together the names of the elements which combine to produce them, and the relative quantities of these elements are indicated either by the use of Latin or Greek prefixes or by variations in the terminal syllables of the names of the elements. End of Chapter 13 Chapter 14 of The Story of Alchemy This LibriVox recording is in the public domain, recording by Peter Yersley. The Story of Alchemy and the Beginnings of Chemistry by M. M. Pattison Muir. Chapter 14 The Modern Form of the Alchemical Quest of the One Thing The study of the properties of the elements shows that these substances fall into groups, the members of each of which are like one another and form compounds which are similar. The examination of the properties and compositions of compounds has shown that similarity of properties is always accompanied by similarity of composition. Hence, the fact that certain elements are very closely allied in their properties suggests that these elements may also be allied in their composition. Now, to speak of the composition of an element is to think of the element as formed by the union of at least two different substances. It implies the supposition that some elements at any rate are really compounds. The fact that there is a very definite connection between the values of the atomic weights and the properties of the elements lends some support to the hypothesis that the substances we call and are obliged at present to call elements may have been formed from one or a few distinct substances by some process of progressive change. If the elements are considered in the order of increasing atomic weights from hydrogen, whose atomic weight is taken as unity because it is the lightest substance known to uranium, an atom of which is 240 times heavier than an atom of hydrogen, it is found that the elements fall into periods and the properties of those in one period vary from element to element in a way which is broadly and on the whole, like the variation of the properties of those in other periods. This fact suggests the supposition, it might be more accurate to say the speculation, that the elements mark the stable points in a process of change, which has not proceeded continuously from a very simple substance to a very complex one, but has repeated itself with certain variations again and again. If such a process has occurred, we might reasonably expect to find substances exhibiting only minute differences in their properties, differences so slight as to make it impossible to assign the substances, definitely and certainly, either to the class of elements or to that of compounds, we find exactly such substances among what are called the rare earths. There are earth-like substances which exhibit no differences of chemical properties, and yet show minute differences in the characters of the light which they emit when they are raised to a very high temperature. The results of analysis by the spectroscope of the light emitted by certain elements at different temperatures may be reasonably interpreted by supposing that these elements are separated into simpler substances by the action on them of very large quantities of thermal energy. The spectrum of the light emitted by glowing iron heated by a Bunsen flame, say at 1200 degrees centigrade, which equals about 2200 degrees Fahrenheit, shows a few lines and flutings. When iron is heated in an electric arc, say to 3500 degrees centigrade, about 6300 degrees Fahrenheit, the spectrum shows some 2000 lines. At the higher temperature, produced by the electric spark discharge, the spectrum shows only a few lines. As a guide to further investigation, we may provisionally infer from these facts that iron is changed at very high temperatures into substances simpler than itself. Sir Norman Lockyer's study of the spectra of the light from stars has shown that the light from those stars which are presumably the hottest, judging by the general character of their spectra, reveals the presence of a very small number of chemical elements, and that the number of spectral lines and therefore the number of elements increases as we pass from the hottest to cooler stars. At each stage of the change from the hottest to cooler stars, certain substances disappear and certain other substances take their places. It may be supposed, as a suggestive hypothesis, that the lowering of stellar temperature is accompanied by the formation, from simpler forms of matter, of such elements as iron, calcium, manganese, and other metals. In the year 1896, the French chemist Becquerel discovered the fact that salts of the metal uranium, the atomic weight of which is 240, and is greater than that of any other element, emit rays which cause electrified bodies to lose their electric charges, and act on photographic plates that are wrapped in sheets of black paper, or in thin sheets of other substances which stop rays of light. The radioactivity of salts of uranium was proved not to be increased or diminished when these salts had been shielded for five years from the action of light by keeping them in leadened boxes. Shortly after Becquerel's discovery, experiments proved that salts of the rare metal thorium are radioactive. This discovery was followed by Madame Curie's demonstration of the fact that certain specimens of pitch blend, a mineral which contains compounds of uranium and of many other metals, are extremely radioactive, and by the separation from pitch blend by Monsieur and Madame Curie, of new substances much more radioactive than compounds of uranium or of thorium. The new substances were proved to be compounds chemically very similar to salts of barium. Their compositions were determined on the supposition that they were salts of an unknown metal closely allied to barium. Because of the great radioactivity of the compounds, the hypothetical metal of them was named radium. At a later time, radium was isolated by Madame Curie. It is described by her as a white, hard, metal-like solid, which reacts with water at the ordinary temperature, as barium does. Since the discovery of radium compounds, many radioactive substances have been isolated. Only exceedingly minute quantities of any of them have been obtained. The quantities of substances used in experiments on radioactivity are so small that they escape the ordinary methods of measurement, and are scarcely amenable to the ordinary processes of the chemical laboratory. Fortunately, radioactivity can be detected and measured by electrical methods of extraordinary fineness. Methods, the delicacy of which very much more exceeds that of spectroscopic methods, than the sensitiveness of these surpasses that of ordinary chemical analysis. At the time of the discovery of radioactivity, about 75 substances were called elements. In other words, about 75 different substances were known to chemists, none of which had been separated into unlike parts, none of which had been made by the coalescence of unlike substances. Compounds of only two of these substances, uranium and thorium, are radioactive. Radioactivity is a very remarkable phenomenon. So far as we know at present, radioactivity is not a property of the substances which form almost the whole of the rocks, the waters, and the atmosphere of the earth. It is not a property of the materials which constitute living organisms. It is a property of some 30 substances. Of course, the number may be increased. A few of which are found widely distributed in rocks and waters, but none of which is found anywhere, except in extraordinarily minute quantity. Radium is the most abundant of these substances, but only a very few grains of radium chloride can be obtained from a couple of tons of pitch blend. In Chapter 10 of the Story of the Chemical Elements, I have given a short account of the outstanding phenomena of radioactivity. For the present purpose, it will suffice to state a few facts of fundamental importance. Radioactive substances are stores of energy, some of which is constantly escaping from them. They are constantly changing without external compulsion, and are constantly radiating energy. All explosives are storehouses of energy which, or part of which, can be obtained from them, but the liberation of their energy must be started by some kind of external shock. When an explosive substance has exploded, its existence as an explosive is finished. The products of the explosion are substances from which energy cannot be obtained. When a radioactive substance has exploded, it explodes again, and again, and again. A time comes sooner or later when it has changed into substances that are useless as sources of energy. The disintegration of an explosive, started by an external force, is generally completed in a fraction of a second. Change of condition changes the rate of explosion. The half-life period of each radioactive substance is a constant characteristic of it. If a gram of radium were kept for about 1,800 years, half of it would have changed into radioinactive substances. Conditions may be arranged so that an explosive remains unchanged. Wet gun-cotton is not exploded by a shock which would start the explosion of dry gun-cotton. In other words, the explosion of an explosive can be regulated. The explosive changes of a radioactive substance which are accompanied by the radiation of energy can not be regulated. They proceed spontaneously in a regular and definable manner, which is not influenced by any external conditions, such as great change of temperature, presence or absence of other substances, so far as these conditions have been made as a subject of experiment. The amount of activity of a radioactive substance has not been increased or diminished by any process to which the substance has been subjected. Explosives are manufactured articles. Explosiveness is a property of certain arrangements of certain quantities of certain elements. So far as experiments have gone, it has not been found possible to add the property of radioactivity to an inactive substance, or to remove the property of radioactivity from an active substance. The cessation of radioactivity of an active substance is accompanied by the disappearance of the substance, and the production of inactive bodies altogether unlike the original active body. Radioactive substances are constantly giving off energy in the form of heat, sending forth rays which have definite and remarkable properties, and producing gaseous emanations which are very unstable, and change some very rapidly, some less rapidly, into other substances, and emit rays which are generally the same as the rays emitted by the parent's substance. In briefly considering these three phenomena, I shall choose radium compounds as representative of the class of radioactive substances. Radium compounds spontaneously give off energy in the form of heat. A quantity of radium chloride which contains one gram of radium continuously gives out per hour a quantity of heat sufficient to raise the temperature of one gram of water through 100 degrees centigrade or 100 grams of water through one degree centigrade. The heat given out by one gram of radium during 24 hours would raise the temperature of 2,400 grams of water through one degree centigrade. In one year the temperature of 876,000 grams of water would be raised through one degree centigrade, and in 1,800 years, which is approximately the half-life period of radium, the temperature of 1,576,800 kilograms of water would be raised through one degree centigrade. These results may be expressed by saying that if one gram, about 15 grains, of radium were kept until half of it had changed into inactive substances, and if the heat spontaneously produced during the changes which occurred were caused to act on water, that quantity of heat would raise the temperature of about 15 and a half tons of water from its freezing to its boiling point. Radium compounds send forth three kinds of rays, distinguished as alpha, beta and gamma rays. Experiments have made it extremely probable that the alpha rays are streams of very minute particles, somewhat heavier than atoms of hydrogen, moving at the rate of about 18,000 miles per second, and that the beta rays are streams of much more minute particles, the mass of each of which is about one-thousandth of the mass of an atom of hydrogen, moving about ten times more rapidly than the alpha particles, that is, moving at the rate of about 180,000 miles per second. The gamma rays are probably pulsations of the ether, the medium supposed to fill space. The emission of alpha rays by radium is accompanied by the production of the inert elementary gas, helium, therefore the alpha rays are, or quickly change into, rapidly moving particles of helium. The particles which constitute the beta rays carry electric charges. These electrified particles, each approximately a thousand times lighter than an atom of hydrogen, moving nearly as rapidly as the pulsations of the ether, which we call light, are named electrons. The rays from radium compounds discharge electrified bodies, ionized gases, that is, cause them to conduct electricity, act on photographic plates, and produce profound changes in living organisms. The radium emanation is a gas about 111 times heavier than hydrogen. To this gas, Sir William Ramsey has given the name niton. The gas has been condensed to a colorless liquid and frozen to an opaque solid, which glows like a minute arc light. Radium emanation gives off alpha particles, that is, very rapidly moving atoms of helium, and deposits exceedingly minute quantities of a solid radioactive substance known as radium A. The change of the emanation into helium and radium A proceeds fairly rapidly. The half-life period of the emanation is a little less than four days. This change is attended by the liberation of much energy. The only satisfactory mental picture which the facts allow us to form at present of the emission of beta rays from radium compounds is that which represents these rays as streams of electrons, that is, particles each about a thousand times lighter than an atom of hydrogen, each carrying an electric charge, and moving at the rate of about 180,000 miles per second, that is, nearly as rapidly as light. When an electric discharge is passed from a plate of metal, arranged as the cathode, to a metallic wire, arranged as the anode, both sealed through the walls of a glass tube or bulb, from which almost the whole of the air has been extracted, rays proceed from the cathode in a direction at right angles there too, and striking the glass in the neighborhood of the anode produce a green phosphorescence. Facts have been gradually accumulated, which force us to think of these cathode rays as streams of very rapidly moving electrons, that is, as streams of extraordinarily minute electrically charged particles, identical with the particles which form the beta rays emitted by compounds of radium. The phenomena of radioactivity, and also the phenomena of the cathode rays, have obliged us to refine our machinery of minute particles by including therein particles at least a thousand times lighter than atoms of hydrogen. The term electron was suggested a good many years ago by Dr. Johnston Stoney for the unit charge of electricity which is carried by an atom of hydrogen, when hydrogen atoms move in a liquid or gas under the directing influence of the electric current. Some chemists speak of the electrons, which are the beta rays from radium, and the cathode rays produced in almost vacuous tubes as non-material particles of electricity. Non-material means devoid of mass. The method by which approximate determinations have been made of the charges on electrons consists in measuring the ratio between the charges and the masses of these particles. If the results of the determinations are accepted, electrons are not devoid of mass. Electrons must be thought of as material particles differing from other minute material particles in the extraordinary smallness of their masses, in the identity of their properties, including their mass, in their always carrying electric charges, and in the vast velocity of their motion. We must think of an electron either as a unit charge of electricity, one property of which is its minute mass, or as a material particle having an extremely small mass, and carrying a unit charge of electricity, the two mental pictures are, almost if not quite, identical. Electrons are produced by sending an electric discharge through a glass bulb containing a minute quantity of air or other gas, using metallic plates or wires as cathode and anode. Experiments have shown that the electrons are identical in all their properties, whatever metal is used to form the cathode and anode, and of whatever gas there is a minute quantity in the bulb. The conclusions must be drawn that identical electrons are constituents of or are produced from very different kinds of chemical elements. As the facts about cathode rays and the facts of radioactivity are, at present, inexplicable, except on the supposition that these phenomena are exhibited by particles of extraordinary minuteness, and as the smallest particles with which chemists are concerned in their everyday work are the atoms of the elements, we seem obliged to think of many kinds of atoms as structures, not as homogeneous bodies. We seem obliged to think of atoms as very minute material particles, which either normally are, or under definite conditions may be, associated with electrically charged particles very much lighter than themselves, all of which are identical, whatever be the atoms with which they are associated, or from which they are produced. In their study of different kinds of matter, chemists have found it very helpful to place in one class those substances which they have not been able to separate into unlike parts. They have distinguished this class of substances from other substances and have named them elements. The expression chemical elements is merely a summary of certain observed facts. For many centuries chemists have worked with a conceptual machinery based on the notion that matter has a grained structure. For more than a hundred years they have been accustomed to think of atoms as the ultimate particles with which they have had to deal. Working with this order-producing instrument they have regarded the properties of elements as properties of the atoms, or of groups of a few of the atoms, of these substances, that they might think clearly and suggestively about the properties of elements and connect these with other chemical facts. They have translated the language of sense perceptions into the language of thought, and for properties of these substances which have not been decomposed have used the more fertile expression, atomic properties. When a chemist thinks of an atom, he thinks of the minutest particle of one of the substances which have the class mark have not been decomposed, and the class name, element. The chemist does not call these substances elements because he has been forced to regard the minute particles of them as undivided, much less because he thinks of these particles as indivisible. His mental picture of their structure as an atomic structure formed itself from the fact that they have not been decomposed. The formation of the class element followed necessarily from observed facts, and has been justified by the usefulness of it as an instrument for forwarding accurate knowledge. The conception of the elementary atom as a particle which had not been decomposed followed from many observed facts beside those concerning elements, and has been justified by the usefulness of it as an instrument for forwarding accurate knowledge. Investigations proved radioactivity to be a property of the very minute particles of certain substances, and each radioactive substance to have characteristic properties among which were certain of those that belong to elements, and to some extent are characteristic of elements. Evidently, the simplest way for a chemist to think about radioactivity was to think of it as an atomic property, hence as atomic properties had always been regarded in the last analysis as properties of elements, it was natural to place the radioactive substance in the class elements, provided that one forgot for the time that these substances have not, the class mark, have not been decomposed. As the facts of radioactivity led to the conclusion that some of the minute particles of radioactive substances are constantly disintegrating, and as these substances had been labeled elements, it seemed probable, or at least possible, that the other bodies which chemists have long, cold elements are not true elements, but are merely more stable co-locations of particles than the substances which are classed as compounds. As compounds can be changed into certain other compounds, although not into any other compounds, a way seemed to be opening which might lead to the transformation of some elements into some other elements. The probability that one element might be changed into another was increased by the demonstration of the connections between uranium and radium. The metal uranium has been classed with the elements since it was isolated in 1840. In 1896, Becquerel found that compounds of uranium and also the metal itself are radioactive. In the light of what is now known about radioactivity, it is necessary to suppose that some of the minute particles of uranium emit particles lighter than themselves and change into some substance or substances different from uranium. In other words, it is necessary to suppose that some particles of uranium are spontaneously disintegrating. This supposition is confirmed by the fact experimentally proved that uranium emits alpha rays, that is atoms of helium, and produces a substance known as uranium X. Uranium X is itself radioactive, it emits beta rays, that is it gives off electrons, in as much as all minerals which contain compounds of uranium, contain compounds of radium also. It is probable that radium is one of the disintegration products of uranium. The rate of decay of radium may be roughly expressed by saying that if a quantity of radium were kept for ten thousand years, only about one percent of the original quantity would then remain unchanged. Even if it were assumed that at a remote time the Earth's crust contained considerable quantities of radium compounds, it is certain that they would have completely disappeared long ago had not compounds of radium been reproduced from other materials. Again, the most likely hypothesis is that compounds of radium are being produced from compounds of uranium. Uranium is a substance which, after being rightly classed with the elements for more than half a century, because it had not been separated into unlike parts, must now be classed with the radium-like substances which disintegrate spontaneously, although it differs from other radioactive substances in that its rate of change is almost infinitively slower than that of any of them except thorium. Footnote that life period of uranium is probably about eight thousand million years. End footnote. Thorium, a very rare metal, is the second of the seventy-five or eighty elements known when radioactivity was discovered, which has been found to undergo spontaneous disintegration with the emission of rays. The rate of change of thorium is considerably slower than that of uranium. Footnote the life period of thorium is possibly about forty thousand million years. End footnote. Reader's note. The author is apparently referring to half lives. End reader's note. None of the other substances placed in the class of elements is radioactive. On page 192 I said that when the radioactive substances had been labeled elements the facts of radioactivity led some chemists to the conclusion that the other bodies which had for long been called by this class name or at any rate some of these bodies are perhaps not true elements but are merely more stable co-locations of particles than the substances called compounds. It seems to me that this reasoning rests on an unscientific use of the term element. It rests on giving to that class name the meaning substances asserted to be undecomposable. A line of demarcation is drawn between elements meaning thereby forms of matter said to be undecomposable but probably capable of separation into unlike parts and true elements meaning thereby groups of identical undecomposable particles. If one names the radioactive substances elements one is placing in this class substances which are specially characterized by a property the direct opposite of that the possession of which by other substances was the reason for the formation of the class. To do this may be ingenious. It is certainly not scientific. Since the time of Lavoisier since the last decade of the 18th century careful chemists have meant by an element a substance which has not been separated into unlike parts and they have not meant more than that. The term element has been used by accurate thinkers as a useful class mark which connotes a property the property of not having been decomposed common to all substances placed in the class and differentiating them from all other substances. Whenever chemists have thought of elements as the ultimate kinds of matter with which the physical world is constructed and they have occasionally so thought and written they have fallen into quagmires of confusion. Of course the elements may someday be separated into unlike parts. The facts of radioactivity certainly suggest some kind of inorganic evolution whether the elements are decomposed is to be determined by experimental inquiry remembering always that no number of failures to simplify them will justify the assertion that they cannot be simplified. Chemistry neither asserts or denies the decomposability of the elements. At present we have to recognize the existence of extremely small quantities widely distributed in rocks and waters of some 30 substances the minute particles of which are constantly emitting streams of more minute identical particles that carry with them very large quantities of energy all of which 30 substances are characterized and are differentiated from all other classes of substances where with chemistry is concerned by their spontaneous mutability and each is characterized by its special rate of change and by the nature of the product of its mutations. We have now to think of the minute particles of two of the 75 or 80 substances which until the other day had not been decomposed and were therefore justly called elements as very slowly emitting streams of minute particles and producing characteristic products of their disintegration and we have to think of some 80 substances as particular kinds of matter at present properly called elements because they are characterized and are differentiated from all other substances by the fact that none of them has been separated into unlike parts. The study of radioactivity has introduced into chemistry and physics a new order of minute particles. Dalton made the atom of beacon light which revealed to chemists paths that led them to wider and more accurate knowledge. Avogadro illuminated chemical and also physical ways by his conception of the molecule as a stable although separable group of atoms with particular properties different from those of the atoms which constituted it. The work of many investigators has made the old paths clearer and has shown to chemists and physicists ways they had not seen before by forcing them to think of and to make use of a third kind of material particles that are endowed with the extraordinary property of radioactivity. Dalton often said thou knowest thou canst not cut an atom but the fact that he applied the term atom to the small particles of compounds proves that he had escaped the danger of logically defining the atom the danger of thinking of it as a particle which never can be cut. The molecule of Avogadro has always been a decomposable particle. The peculiarity of the new kind of particles the particles of radioactive bodies is not that they can be separated into unlike parts by the action of external forces but that they are constantly separating of their own accord into unlike parts and that their spontaneous disintegration is accompanied by the production of energy the quantity of which is enormous in comparison with the minuteness of the material specs which are the carriers of it. The continued study of the properties of the minute particles of radioactive substances a new name is needed for those most mutable of material grains must lead to discoveries of great moment for chemistry and physics that study has already thrown much light on the phenomena of electric conductivity it has given us the electron a particle at least a thousand times lighter than an atom of hydrogen it has shown us that identical electrons are given off by or are separated from different kinds of elementary atoms under definable conditions it has revealed unlooked for sources of energy it has opened and begun the elucidation of a new department of physical science it has suggested a new way of attacking the old problem of the alchemists the problem of the transmutation of the elements the minute particles of two of the substances for many years classed as elements give off electrons uranium and thorium are radioactive electrons are produced by sending an electric discharge through very small traces of different gases using electrodes of different metals electrons are also produced by exposing various metals to the action of ultraviolet light and by raising the temperature of various metals to incandescence electrons are always identical whatever be their source three questions suggest themselves can the atoms of all the elements be caused to give off electrons are electrons normal constituents of all elementary atoms are elementary atoms co-locations of electrons these questions are included in the demand is it possible to imagine a model which has in it the potentiality of explaining radioactivity and other allied phenomena as well as all other chemical and physical properties of elements and compounds these questions are answerable by experimental investigation and only by experimental investigation if experimental inquiry leads to affirmative answers to the questions we shall have to think of atoms as structures of particles much lighter than themselves we shall have to think of the atoms of all kinds of substances however much the substances differ chemically and physically as co-locations of identical particles we shall have to think of the properties of atoms as conditioned in our final analysis by the number and arrangement of their constitutive electrons now if a large probability were established in favor of the view that different atoms are co-locations of different numbers of identical particles or of equal numbers of differently arranged identical particles we should have a guide which might lead to methods whereby one co-location of particles could be formed from another co-location of the same particles a guide which might lead to methods whereby one element could be transformed into another element to attempt to imagine a model which has in it the potentiality of explaining radioactivity the production of cathode rays and the other chemical and physical properties of elements and compounds might indeed seem to be a hopeless undertaking a beginning has been made in the mental construction of such a model by professor Sir J. J. Thompson to attempt a description of his reasoning and his results is beyond the scope of this book the facts that the emanation from radium compounds spontaneously gives off very large quantities of energy and that the emanation can easily be brought into contact with substances on which it is desired to do work suggested to Sir William Ramsey that the transformation of compounds of one element into compounds of another element might possibly be affected by enclosing a solution of a compound along with radium emanation in a sealed tube and leaving the arrangement to itself under these conditions the molecules of the compound would be constantly bombarded by a vast number of electrons shot forth at enormous velocities from the emanation the notion was that the molecules of the compound would break down under the bombardment and that the atoms so produced might be knocked into simpler groups of particles in other words changed into other atoms by the terrific silent shocks of the electrons fired at them incessantly by the disintegrating emanation Sir William Ramsey regards his experimental results as establishing a large probability in favor of the assertion that compounds of copper were transformed into compounds of lithium and sodium and compounds of thorium of cerium and of certain other rear metals into compounds of carbon the experimental evidence in favor of this statement has not been accepted by chemists as conclusive a way has however been opened which may lead to discoveries of great moment let us suppose that the transformation of one element into another element or into other elements has been accomplished let us suppose that the conception of elementary atoms as very stable arrangements of many identical particles from about a thousand to about a quarter of a million times lighter than the atoms has been justified by crucial experiments let us suppose that the conception of the minute grains of radioactive substances at particular but constantly changing arrangements of the same identical particles stable groups of which are the atoms of the elements has been firmly established one result of the establishment of the electronic conception of atomic structure would be an increase of our wonder at the complexity of nature's ways and an increase of our wonder that it should be possible to substitute a simple almost rigid mechanical machinery for the ever-changing flow of experience and by the use of that mental mechanism not only to explain very many phenomena of vast complexity but also to predict occurrences of similar entanglement and to verify these predictions the results which have been obtained in the examination of radioactivity of cathode rays of spectra at different temperatures and of phenomena allied to these bring again into prominence the ancient problem of the structure of what we call matter is matter fundamentally homogeneous or heterogeneous chemistry studies the relations between the changes of composition and the changes of properties which happens simultaneously in material systems the burning fire of wood coal or gas the preparation of food to excite and to satisfy the appetite the change of minerals into the iron steel copper brass lead tin lighting burning and lubricating oils dyestuffs and drugs of commerce the change of the skins wool and hair of animals and of the seeds and fibers of plants into clothing for human beings the manufacture from rags grass or wood of a material fitted to receive and to preserve the symbols of human hopes fears and aspirations love and hate pity and aversion the strange and most delicate processes which happening without cessation in plants and animals and men maintain that balanced equilibrium which we call life and when the silver cord is being loosed and the bowl broken at the system the awful changes which herald the approach of death not only the growing grass in mid summer meadows not only the coming of autumn in diet garments traveling in the glory of his apparel but also the opening buds the pleasant sense the tender colors which stir our hearts in the springtime the only pretty ring time when birds do sing ding-a-dong ding these and the thousand other changes have all their aspects which it is the business of the chemist to investigate confronted with so vast a multitude of never ceasing changes and bidden to find order there if he can bidden rather compelled by that imperious command which forces the human mind to seek unity and variety and if needs be to create a cosmos from a chaos no wonder that the early chemists jumped at the notion that there must be that there is some one thing some universal essence which binds into an orderly hole the perplexing phenomena of nature some water of paradise which is for the healing of all disorder some well at the world's end a draft whereof shall bring peace and calm security the alchemists set forth on the quest their quest was barren they made the great mistake of fashioning the one thing the essence the water of paradise from their own imaginings of what nature ought to be in their own likeness they created their goal and the roads to it if we are to understand nature they cried her ways must be simple therefore her ways are simple chemists are people of a humbler heart their reward has been greater than the alchemists dreamed by selecting a few instances of material changes and studying these with painful care they have gradually elaborated a general conception of all those transformations wherein substances are produced unlike those by the interaction of which they are formed that general conception is now both widening and becoming more definite today chemists see a way opening before them which they reasonably hope will lead them to a finer a more far-reaching a more suggestive at once a more complex and a simpler conception of material changes than any of those which have guided them in the past the end of the story of alchemy and the beginnings of chemistry