1825.] of Claude-Louis Berthollet. 169 



and combine with both acids, in the ordinary compound ratio of 

 their affinity and quantity ? 



Berthollet's explanation of this seeming anomaly was exceed- 

 ingly luminous, and indeed constitutes the leading characteristic 

 of his theory. The force, said he, which produces combination 

 among substances whose constitution is different, and which is 

 usually styled Chemical Affinity, is merely one of the effects or 

 modes of action of the general principle of molecular attraction : 

 another, no less extensive and powerful, is the influence of this 

 principle in producing combination between particles of a similar 

 constitution. This latter force, we have been accustomed to 

 call Co/iesio7i: and it has been too frequently regarded as a 

 power sui generis, as a physical in contradistinction to a chemical 

 power of matter, and one which is annihilated the instant it is 

 overcome. On the contrary, as is the case with every other 

 compressed natural force, it continues to act even after, by the 

 intervention of some more powerful principle, the particles of 

 the homogeneous solid have been completely disunited. 



Hence in every case of chemical combination and decomposi- 

 tion, the affinity of cohesion and the affinity of combination 

 must constitute direct antagonists to one another's action ; and 

 when two substances are placed in a situation favourable to 

 chemical action, they will either remain unaltered or a combina- 

 tion will take place, according as either of these two forces pre- 

 dominates in mtensity over the other. According to this theory 

 it should be observed, decomposition must be proscribed from 

 the list of the causes which tend to produce chemical changes : 

 they are invariably the consequences of combination. Thus, in 

 one of the illustrations already adduced, the cohesive affinity of 

 the constituents of the oxalate of hme, is more than sufficient to 

 counterbalance both its tendency to combine with water, and 

 the affinity of the acetic acid for the lime : it consequently pre- 

 cipitates, and leaves the acetic acid in a disengaged state in the 

 liquid. 



Another important circumstance which modifies the affinity of 

 combination is elasticity. Many substances acquire such a 

 tendency to expansion by combining with caloric (which is the 

 cause of expansibility), that t'.iey become no longer obedient 

 either to the affinity of combination or of cohesion: whenever, 

 therefore, their expansibility is sufficiently augmented by the 

 accumulation of heat, or when the affinity by which they were 

 held in combination is weakened by the intervention of a third 

 body, ihey quit the solid or liquid in which they had previously 

 existed condensed, and assume the form of an elastic gas or 

 vapour. 



Such is a very general outline of Berthollet's theory of 

 affinity ; but it would be impossible within the limits to which 

 we are necessarily restricted, to convey an adequate conception 



