Faraday found that the quantity of hydrogen which in the above experiment is transferred to the cathode in a given time is proportional to the quantity of electricity transferred in the same time. A gram of hydrogen always takes the same amount of electricity with it. By experiment this amount of electricity can be determined, and, since the weight in grams of the hydrogen atom is known, it is possible to calculate the amount of one atom. In electrostatic units it is 4·77 × 10⁻¹⁰, _i.e._, 477 billionth[1] parts. A chlorine atom then carries with it 4·77 × 10⁻¹⁰ electrostatic units of negative electricity. Since its atomic weight is 35·5, then 35·5 grams of chlorine will take as much electricity as 1 gram of hydrogen. The ratio _e_/_m_ between the charge _e_ and the mass _m_ is then 35·5 times as great for hydrogen as for chlorine. [1] Billion used here to mean one million million, and trillion to mean one million billion. We have temporarily restricted ourselves to the electrolysis of hydrogen chloride. Let us now assume that we have chloride of zinc (ZnCl₂), which, by electrolysis, is separated into chlorine and zinc. Each atom of chlorine will, as before, carry 4·77 × 10⁻¹⁰ units of negative electricity to the anode; but since zinc is divalent (cf. p. 17) and one atom of zinc is joined to two of chlorine, therefore one atom of zinc must carry a charge of 2 × 4·77 × 10⁻¹⁰ units of positive electricity to the cathode. An atom or a group of atoms, with valence of three, in electrolysis carries 3 × 4·77 × 10⁻¹⁰ units, etc. We see then, that the quantity of electricity which accompanies the atoms in electrolysis is always 4·77 × 10⁻¹⁰ electrostatic units or an integral multiple thereof. This suggests the thought that electricity is atomic and that the quantity 4·77 × 10⁻¹⁰ units is the smallest amount of electricity which can exist independently, _i.e._, the _elementary quantum of electricity_ or the “atom of electricity.” The atom of a monovalent element, when charged or ionized, should have one atom of electricity; a divalent, two, etc. On the two-fluid theory it was most reasonable to assume that there were two kinds of atoms of electricity representing, respectively, positive and negative electricity. In Fig. 15 there is given, in accordance with the two-fluid theory, a rough picture of a chlorine ion and a hydrogen ion and their union into a molecule. [Illustration: FIG. 15.—Provisional representation (according to the two-fluid theory) of A, a hydrogen ion; B, a chlorine ion; and C, a molecule of hydrogen chloride.] The atoms of electricity seemed to differ essentially from the usual atoms of the elements in their apparent inability to live independently; they seemed to exist only in connection with the atoms of the elements. They would seem much more real if they could exist independently. That such existence really is possible, has been discovered by the study of the motion of electricity in gases. Vacuum Tube Phenomena.