 In the nineteenth century it was well understood that the chemistry of substances consisted of atoms, but we knew very little about atoms themselves. It was the discovery of the electron by J. J. Thompson that first introduced the idea that an atom had parts. In 1898, with the electron being so light compared to the atom, Thompson suggested what is called the plum pudding model of the atom, with a uniform mass of positively charged matter containing spots of electrons embedded in it like plums in a pudding. A way to find out if this model is correct or not is to probe the pudding, but you need to probe with something smaller than the object being probed. For example, you can't probe a grain of sand with your finger. In 1898 there simply wasn't anything smaller than atoms that could be used to probe an atom.