Note 210 — Chapter XI

Comptes Rendus de l’Académie des Sciences, 1840, No. 5. A good translation of Dumas’s Memoir appeared in the Philosophical Magazine, from which I extract the following familiar exposition of the laws of substitution:—“Let me make a comparison drawn from a familiar order of ideas. Let us put ourselves in the place of a man overlooking a game at chess without the slightest knowledge of the game. He would soon remark that the pieces must be used according to positive rules. In chemistry, the equivalents are our pieces, and the law of substitutions one of the rules which preside over their moves. And as in the oblique move of the pawns one pawn must be substituted for another, so in the phenomena of substitution one element must take the place of another. But this does not hinder the pawn from advancing without taking anything, as the law of substitution does not hinder an element from acting on a body without displacing or taking the place of any other element that it may contain.”—Memoir on the Law of Substitutions, and Theory of Chemical Types.