The question next arises, is the valency of an
element fixed or variable? If the word be defined as above, it is
absolutely certain that the valency varies. Thus, tin may be trivalent,
SnCl_{2}, or tetravalent, SnCl_{4}. Accordingly elements have been
classed as monads, dyads, triads, etc. The lecturer objected most
strongly to the word "atomicity;" he could not conceive of one atom
being more atomic than another; he could understand the atomicity of
a molecule or the equivalency of an atom, but not the atomicity of an
atom; the expression seemed to him complete nonsense. He next considered
the possibility of assigning a fixed limit to this valency or adicity of
an atom, and concluded that the adicity was not absolutely fixed, but
was fixed in relation to certain elements, e.g., C never combines with
more than four atoms of H; O never more than two atoms of H, etc. The
adicity of an element when combined with two or more elements is usually
higher than when combined with only one, e.g., NH_{3}, NH_{4}Cl. The
term "capacity of saturation," may be used as a synonym for adicity, if
care be taken to distinguish it from other kinds of saturation, such as
an acid with an alkali, etc.
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