190 M. Dumas on the Lam of Substitutions, 



tions of condition which the general formula ought to satisfy. 

 Now it is evident that the metamorphosing actions are often 

 nothing but means for operating substitutions, by taking ad- 

 vantage of affinities more complicated than those which are 

 made use of in the ordinary substitutions. 



It is therefore more than ever requisite to apply to the 

 study of the actions of bodies, and not to trouble ourselves 

 about the distance which often separates the point from which 

 we started from that which we reach ; for it may well happen, 

 that these two points, so unlike in their properties, are really 

 united to each other by the theory of substitutions, and belong 

 to the same molecular grouping. 



The law of substitutions expresses, then, a simple experi- 

 mental relation ; it is limited to the expression of a relation 

 often observed between the hydrogen lost and the chlorine 

 absorbed, by a hydrogenated body submitted to the action 

 of chlorine. This law establishes only, that if the substance 

 loses 1, 2, 3 equivalents of hydrogen, it will gain 1, 2, 3 equi- 

 valents of chlorine ; but it does not explain this fact. 



The theory of types goes further; it explains what the law 

 of substitutions is content to determine. It considers organic 

 bodies as being formed of particles, which may be displaced 

 and have their places supplied by others without the body 

 being destroyed, so to speak. In the cases above-quoted, the 

 molecule of acetic acid, that of aether, may lose hydrogen and 

 take chlorine, without ceasing to constitute an acid or basic 

 molecule, formed of the same number of equivalents and en- 

 dowed with the same number of fundamental properties. 



It is then because, that on pain of being destroyed, the mo- 

 lecule of acetic acid must take an equivalent of chlorine to 

 stand for the equivalent of hydrogen which it loses, that this 

 substitution, this remplacement is effected. Thus it is that the 

 theory of types explains the law of substitutions. 



The substitution of one element for another, equivalent for 

 equivalent, is the effect ; the preservation of the type is the 

 cause. The organic molecule, the organic type, constitute an 

 edifice, in which a course {assise) of hydrogen can have its 

 place supplied by a course of chlorine, of bromine, or of oxy- 

 gen, without the exterior relations of that edifice being thereby 

 modified. But it is necessary, when the course of hydrogen is 

 taken away, to put something in its place ; if not, the edifice 

 crumbles or is transformed. 



The law of substitutions was hardly put forth before it be- 

 came the subject of severe criticisms in Germany, to which I 

 thought it useless to reply. If this law was just, it was for 



