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The meaning attached to the word "valency," is simply one of interchangeability, just as we say a penny is worth two halfpennies or four farthings. 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.
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.
Even though it may presently be torn down, it is for the time being a completed structure; and a consideration of the valency of its atoms gives the best clew that has hitherto been obtainable as to the character of its architecture.
While, however, an appreciation of this ceaseless activity of the atom is essential to a proper understanding of its chemical efficiency, yet from another point of view the "saturated" molecule that is, the molecule whose atoms have their valency all satisfied may be thought of as a relatively fixed or stable organism.
The terms "chemical appetite" and "specific chemical appetite" are names I have coined for your present benefit, but for which chemists would use the words "affinity" and "valency" respectively.
Just why different elements should differ thus in valency no one as yet knows; it is an empirical fact that they do. And once the nature of any element has been determined as regards its valency, a most important insight into the possible behavior of that element has been secured.
Tin is usually said to be dyad in stannous compounds and a tetrad in stannic compounds, but in a compound like SnCl AmCl, is not tin really a tetrad? and yet it is a stannous compound, and gives a black precipitate with H S; so that valency does not necessarily go with the series.
But no inkling of an explanation of these strange variations of molecular structure came until the discovery of the law of valency. Then much of the mystery was cleared away; for it was plain that since each atom in a molecule can hold to itself only a fixed number of other atoms, complex molecules must have their atoms linked in definite chains or groups.
It is the river Valency of which he speaks, the more important of the streams that join just above the haven. This is a tiny land-locked harbour with stone piers, at which some coal, lime, and general merchandise are imported; the entrance is very difficult to make, and vessels that succeed in doing so have to be warped in by immense hawsers.
But this new chemistry has grown up by the help of hypotheses, such as those of Dalton and of Avogadro, and that singular conception of 'bonds' invented to colligate the facts of 'valency' or 'atomicity, the first of which took some time to make its way; while the second fell into oblivion, for many years after it was propounded, for lack of empirical justification.
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