VALENCY
We must now explain one or two terms which are extremely important for understanding what is to follow. The first of these is Valency. We know that chemical combinations take place in fixed proportions by weight; this is known as the “Constancy of Composition.” There is always an equivalence noted. This doctrine of equivalence is merely the numerical expression of the definiteness of chemical change. Calculations are made from the point-of-view of combining with a unit-weight of hydrogen (the Unit element). In chemical compounds, then, the doctrine of equivalence says that these atomic weights represent quantities of different elementary substances which are of the same chemical value as measured by their capacity for displacing the same weight of hydrogen.
A new property of the atom is thus brought out, viz., its value as measured by the number of atoms with which it can combine. This property is appropriately described as the “Valency” of the atom. If the atomic weight contains the equivalent once, i. e., if the equivalent and atomic weight are identical, that atom can combine only with one atom of hydrogen, or of chlorine, bromine, etc. The formulæ of the compounds, HCl, HBr, etc., expresses this fact. If the equivalent is contained twice in the atomic weight, then that atom can obviously combine with two atoms of hydrogen, chlorine, etc.; if it is contained three times in the atomic weight, the combining capacity or valency of the atom is three; and so forth.