Page:Elementary Text-book of Physics (Anthony, 1897).djvu/340

336 ions. To illustrate, we know that when hydrogen is evolved from hydrochloric acid, HCl, its ion is univalent. Now when it is evolved from water, H2O, we may either consider the H2 as a bivalent ion or as two univalent ions. Similarly we may consider the O as a bivalent ion or as two univalent ions, though it can never be actually broken up into two such ions. We may consider a molecule, then, as made up either of two n-valent ions or of 2n univalent ions. The weight of each of the n-valent ions may be measured in terms of the weight of the hydrogen atom taken as a unit, and is the molecular weight of the ion. This weight divided by the valency n is the weight of the univalent ion. It may be called the ionic weight.

Now the passage of a current through different electrolytes evolves their constituents in amounts proportional to their molecular weights divided by their valencies. It therefore evolves the ions in proportion to their ionic weights, or it evolves the same number of univalent ions in each electrolyte. Faraday's two laws may therefore be summed up in the statement that the number of univalent ions evolved by a current in any electrolyte is proportional to the quantity of current.

By this mode of considering electrolysis, we are led to the conclusion that each pair of univalent ions liberated during electrolysis is associated with a pair of charges numerically equal and of opposite sign. These charges are called ionic charges. An n-valent ion is associated with n ionic charges. If we use the conception of tubes of force, each positive univalent ion may be considered as the origin of a tube of force which terminates on a negative ion. Since the ionic charges are all equal, these tubes may be taken as unit tubes, which are no longer defined arbitrarily, but are based upon a constant of Nature.

281. The Voltameter.—These laws were used by Faraday to establish a method of measuring current by reference to an arbitrary standard. The method employs a vessel containing an electrolyte in which suitable electrodes are immersed, so arranged that the products of electrolysis, if gaseous, can be collected and