and the Theory of Chemical Types. 327 



The law of substitutions sees then in these five compounds 

 some of the nearest, the most necessary modifications of ether. 

 The theory of equivalents sees there any modifications what- 

 ever more or less possible. The one says, these five bodies 

 must be formed first very easily and very abundantly; the 

 other says, they may be termed, and many others with them. 



If the acetic acid is in question, the theory of equivalents 

 would besides announce the possible formation of compounds 

 so numerous, that nothing could guide the observer. The 

 law of substitutions, more precise, foresees and predicts that 

 in losing 1, 2, '3 equivalents of hydrogen, the acetic acid will 

 take 1, 2, 3 equivalents of chlorine, and will thus produce 

 three new compounds. One of them constitutes the so-called 

 chloracetic acid. 



In a multitude of possible actions which are nearly equally 

 foreseen by the theory of equivalents, the law of substitutions 

 discovers with certainty those which are about to be produced; 

 it foresees them, predicts them, and up to tlie present time its 

 help has truly been of invaluable efficacy. 



How, without it, should we have been led to discover, one 

 after the other, four or five mixed products, hardly differing 

 from each other, in some actions recently studied ? Other- 

 wise, how would it have been possible to perceive, that the 

 action which was to be produced had not been exhausted, if 

 the formulae, through the impossibility of making them agree 

 with the law of substitutions, had not warned the observer? 



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 having the slightest knowledge of 

 the game. He would soon remark, that the pieces must be 

 used according to positive rules. In chemistry, the equiva- 

 lents are our pieces, and the law of substitutions one of the 

 rules which presides over their moves. And as in the oblique 

 move of the pawns, one pawn must be substituted for another, 

 so, in the phaenomena 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 substi- 

 tutions does not hinder an element from acting on a body 

 without displacing or taking tiie place of any other element 

 which it may contain. 



How can we believe that a knowledge of the rules which 

 govern the game of our chess-board, is useless for the ex- 

 planation of the moves which offer themselves, for the purpose 

 ot foreseeing those which are about to arise from the rela- 

 tions of the different pieces, of the various agents placed in 

 contact ? 



