502 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



supposition that elective affinity is an invariable force, and 

 of such a nature that a body which expels another from its 

 combination, cannot possibly be separated from the same 

 by the body which it eliminated " (loc. at. pp. 3-4). 



" It is my purpose to prove in the following sheets, that 

 elective affinity, in general, does not act as a determinate 

 force, by which one body separates completely another from 

 a combination ; but that, in all the compositions and 

 decompositions produced by elective affinity, there takes 

 place a partition of the base, or subject of the combination, 

 between the two bodies whose actions are opposed ; and 

 that the proportions of this partition are determined, not 

 solely by the difference of energy in the affinities, but also 

 by the difference of the quantities of the bodies ; so that an 

 excess of quantity of the body whose affinity is the weaker, 

 compensates for the weakness of affinity " (loc. cit. pp. 45). 



" I shall prove . . . that, in opposing the body A to the 

 combination BC, the combination AC can never take place ; 

 but that the body C will be divided between the bodies A 

 and B, proportionally to the affinity and quantity of each " 

 (loc. cit. p. 6). 



Berthollet illustrates the law of mass-action. Several 



examples, of which two may be quoted, are given of the 



influence of mass in compensating for weakness of 

 affinity. 



" I have kept an equal quantity of potash, and of sulphate 

 of barytes, in a small quantity of boiling water. The potash 

 had been prepared by alcohol, 1 and contained no carbonic 

 acid . . . The operation was performed in a retort, and . . . 

 was continued until the mixture was desiccated : the residue 

 was washed with alcohol, which dissolved the potash, and 

 after that with water, which also produced an alkaline 

 solution, the alkali of which I saturated with acetic acid ; 

 after which, by evaporation, the solutions yielded crystals 

 possessing all the characters and qualities of the sulphate of 

 potash. Whence it appears that the sulphate of barytes was 



1 This dissolves caustic potash, but leaves the carbonate, sulphate, 

 &c., undissolved. 



