In the case of some substances, such as _salt_, the molecule is very simple and composed of two atoms. In other substances, such as albumen or white of egg, the molecules are very complicated and composed of hundreds of atoms. The word _atom_ means something which “_cannot be cut_,” and until comparatively recent time the opinion was held that atoms of matter were the smallest indivisible portions of matter which could exist. More than twenty-five years ago, Sir William Crookes showed, by numerous beautiful experiments, that in a vacuum tube, such as you have seen used to-day, a torrent of small particles is projected from the negative terminal when an electric current is passed through the tube. This stream of particles is called the cathode stream, or the cathode radiator. Within recent times, Sir Joseph Thomson has furnished a proof that this cathode stream consists of particles very much smaller than chemical atoms, each particle being charged with negative electricity. These particles are now called _corpuscles_, or _electrons_. It has been shown that these electrons are constituents of chemical atoms, and when we remove an electron from an atom we leave the remainder positively electrified. An atom can, therefore, by various means be divided into two portions of unequal size. First, a very small part which is charged with negative electricity, and, secondly, a remaining larger portion charged with positive electricity. These two parts taken together are called ions, _i.e._ wanderers. The negative ions, or electrons, or corpuscles, taken together constitute what we call negative electricity, and up to the present no one has been able to show that the corpuscle can be unelectrified. Hence the view has been expressed that what we call electricity is a kind of matter, atomic in structure, and that these negative ions or corpuscles collectively are, in fact, the atoms of the electric fluid. These corpuscles can move freely in the interior of some solids, moving between the molecules of the solid just as little dogs can run about in and amongst a crowd of people in a street. In these cases the substance is called a conductor of electricity. In other substances the movement of the corpuscles is more restricted, and these constitute the various kinds of so-called non-conductors.