One thing I am at liberty to tell you is that my fellow-electrons who are locked up within the atoms are not without hope that they may gain their freedom once more at some future time. I know this first-hand, for I have met some fellow-electrons who have escaped from within an atom, but I shall delay telling you about these fellows till the succeeding chapter. My object in mentioning this fact now is to give you confidence in what I am about to say regarding the nature of the atom. On one occasion I overheard a conversation between two men who were discussing the construction of matter. One remarked that the atoms were the bricks of the universe, whereupon the other asked how the little bricks were cemented together. I wish that man could have seen a lump of matter as we see it. He would have been surprised to learn that the atoms never really touch each other. They are always surging to and fro, or _vibrating_, and it is this motion which constitutes the _temperature_ of the body which they compose. It must be clear, however, that in a solid body one atom attracts another atom across the intervening atomic spaces. This is another duty devolving upon us; what we do, really, is to upset the electric balance between the different atoms, and thus produce electrical attraction. First of all, perhaps, I should explain that the different kinds of atoms are simply congregations of different numbers of electrons. Of course there is the other part, of which I am forbidden to speak--the part which man vaguely describes as _positive electricity_. However, you may take it from me that while it is true that the main difference between an atom of gold and an atom of iron, or of oxygen, is in the number of electrons it contains, there is a very important difference in the arrangement of the electrons. You know that they form rings outside one another, all of which revolve at enormous speeds. The number of electrons in the different rings varies according to the kind of atom. It is quite correct for man to speak of the atoms containing certain definite numbers of electrons, but I should like you to understand clearly that the exact number of electrons is not permanently fixed; one or more electrons can slip off one atom and become attached to a neighbouring atom which happens to be capable of accepting it or them. It is the interchange of these few detachable electrons that causes one atom to attract another. In other words, it is the differently charged atoms which attract each other, just as man crowds a surplus of electrons on to one object and finds it attracted bodily towards another object having a deficiency of electrons.