is proportional to the quantity of matter
deposited on the electrodes. This leads us at once to the
consideration that, in any given solution, all the ions possess
individual charges equal in absolute value.

The second law may be stated in these terms: an atom-gramme of metal
carries with it into electrolysis a quantity of electricity
proportionate to its valency.[19]

[Footnote 19: The valency or atomicity of an element may be defined as
the power it possesses of entering into compounds in a certain fixed
proportion. As hydrogen is generally taken as the standard, in
practice the valency of an atom is the number of hydrogen atoms it
will combine with or replace. Thus chlorine and the rest of the
halogens, the atoms of which combine with one atom of hydrogen, are
called univalent, oxygen a bivalent element, and so on.--ED.]

Numerous experiments have made known the total mass of hydrogen
capable of carrying one coulomb, and it will therefore be possible to
estimate the charge of an ion of hydrogen if the number of atoms of
hydrogen in a given mass be known. This last figure is already
furnished by considerations derived from the kinetic theory, and
agrees with the one which can be deduced from the study of various
phenomena. The result is that an ion of hydrogen having a mass of 1.3
x 10^{-20} grammes bears a charge of 1.3 X 10^{-20} electromagnetic
units; and the second law will immediately enable the charge of any
other ion to be similarly estimated.

The measurements of conductivity, joined to certain considerations
relating to the differences of concentration which appear round the
electrode in electrolysis, allow the speed of the ions to be
calculated. Thus, in a liquid containing 1/10th of a hydrogen-ion per
litre, the absolute speed of an ion would be 3/10ths of a millimetre
per second in a field where the fall of potential would be 1 volt per
centimetre. Sir Oliver Lodge, who has made direct experiments to
measure this speed, has obtained a figure very approximate to this.
This value is very small compared to that which we shall meet with in
gases.

Another consequence of the laws of Faraday, to which, as early as 1881,
Helmholtz drew attention, may be considered as the starting-point of
certain new doctrines we shall come across later.

Helmholtz says: "If we accept the hypothesis that simple bodies are
composed of atoms, we are obliged to admit that, in the same way,
electricity, whether positive or n