504 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



divides itself between the two bases. It also appears, that 

 acids may be partially separated from their bases by other 

 acids, whose affinities were supposed to be weaker; in 

 which case, the base is divided between the two acids " 

 (loc. tit. p. 1 1 ). 



Mass-action. Berthollet's method for detecting the 

 reversed action in the cases quoted above depended on 

 using a solution of potash which was concentrated by 

 evaporating finally to dryness. In this way the action of 

 mass, which was the chief subject of his research, was 

 made use of to the fullest extent. On this point he writes : 



" If but a small quantity of the decomposing substance be 

 employed, the effect will not be perceptible ; but if, on the 

 contrary, a large quantity be employed, as for instance, if 

 I had treated the sulphate of barytes successively with 

 additional quantities of potash, and removed, by repeated 

 washing, the disengaged barytes, I should have ultimately 

 decomposed the sulphate of barytes almost entirely. 1 The 

 greater then the relative quantity of the decomposing 

 substance, the greater will be the effect produced " (loc. tit. 

 p. 12). 



Effect of precipitation on balanced actions. In consider- 

 ing balanced actions in solution, Berthollet recognised that the 

 action of mass was seriously limited in the case of insoluble 

 substances, since " If an insoluble substance be opposed to 

 a combination, it is evident that only a very small quantity 

 of it can act " (loc. cit. p. 32). This is expressed in modern 

 terms by saying that the " active mass " of a solid is constant, 

 no further increase of activity being produced by increas- 

 ing the quantity of solid beyond the point at which it 

 suffices to saturate the liquid. 



When one acid is being competed for by two bases or 

 one base by two acids, the result is influenced mainly by 

 the relative strengths of the competing bases or of the com- 

 peting acids. But when two acids are present in sufficient 

 1 In order to do this, carbonate of potash must be used. 



