The discovery of the atomic nature of electricity came later.  Faraday established the fact that in certain processes there was more than a hint that electricity was always present in multiples of a definite unit.  In the process called electrolysis the electric current is driven across a cell full of liquid containing molecules of some substance.  When the electricity passes there is a loosening of the bonds that bind together the atoms of the molecule, and a separation; atoms of one kind travel with the electricity across the cell and are deposited where the current leaves the cell; the other kind travel the opposite way.  In this way for example we deposit silver on metal objects in electro-plating processes, or separate out the purest copper for certain electrical purposes.  The striking thing which Faraday discovered was that the number of atoms deposited always bore a very simple relation to the quantity of electricity that passes.  The same current passing in succession through cells containing different kinds of molecules broke up the same number of molecules in each cell.  It was as if in each electrolytic cell atoms of matter and atoms of electricity travelled together.  The movement of an atom meant the simultaneous movement of a definite quantity of electricity.  Electricity was, so to speak, done up in little equal parcels, and an atom of matter on the move, which was termed an ion, or wanderer, carried, not a vaguely defined amount of electricity, but one of these definite parcels.

It was not, however, until the later years of the nineteenth century that the natural unit of electricity was manifested by itself and without a carrier.  At a famous address to the British Association at York in 1881 Sir William Crookes described the first marvellous experiments in which this feat had been accomplished, though there was still to come a long controversy before the interpretation was clearly accepted.  It is now definitely established that there is a fundamental atom of electricity which we now call the electron.  As we all know electrification is of two kinds—­a positive and a negative.  The electron is of the negative kind.  There does not appear to be a corresponding positive atom of electricity, or at least not one that is so singular in its properties as the electron.  Electrons go to the making of all atoms, just as atoms go to the making of molecules.  The atom which is neutral, that is, shows neither positive nor negative electrification, must contain positive electricity in some form to balance the electrons which we know it contains.  When we strip an atom, as we know how to do, of one or more of these electrons, the remainder is positively charged.  The positive ion is any sort of an atom or molecule which has become positively electrified in this way.  An atom which has become positive by the loss of one or more of its electrons exercises a force on any spare electrons in its neighbourhood or on any atom carrying a spare electron.  When there are large numbers of atoms seeking in this way to become neutral once more, as occurs often in Nature, the forces generated may be tremendous.  They are shown, for example, in the lightning-stroke.  But indeed it would seem that all the chemical forces of which we have already spoken depend ultimately upon the electric state of the atom concerned.