508 HISTORICAL INTRODUCTION TO CHEMISTRY CHAP. 



lime, a precipitate of oxalate of lime is obtained, much less 

 abundant than if a solution of a neutral oxalate had been 

 used, because the action of the acid allows only part of the 

 oxalate of lime to be formed, but with an oxalate this 

 obstacle would not have existed" (Chemical Statics, I. 67). 

 Here the action 



H 2 C 2 O 4 + CaCl 2 ^CaC 2 O 4 + 2HC1 



(insoluble) 



is reversible, since the insolubility of its calcium salt enables 

 the oxalic acid to rob the stronger muriatic acid of part of 

 its lime. 



Interesting cases of the same sort are found amongst the 

 sulphides, e.g. : 



CuS0 4 + H 2 S -> CuS + H 2 S0 4 



(precipitate) 



ZnSO 4 + H 2 S <- ZnS + H 2 SO 4 . 



(dissolves) 



In the former case the insolubility of the sulphide causes it 

 to be precipitated, in opposition to the tendency, which is 

 dominant in the latter case, for the stronger acid to secure 

 possession of the base. 



Influence of vaporisation on balanced actions. In 

 reference to the influence of gases on balanced actions, 

 Berthollet writes : 



" When a substance assumes the state of gas, on separating 

 from an intimate combination, it becomes elastic, and can 

 oppose no further resistance to the decomposing action : 

 whence it appears that substances of this nature do not 

 act by their mass. The decomposing substance can then 

 effect a complete decomposition ; and it will suffice to 

 employ just as much of it as would have been necessary to 

 form the same combination immediately, or at least a very 

 trifling excess." 



" Thus carbonic acid may be disengaged from its combina- 

 tion by another substance, whose affinity for the base of the 

 carbonate might be less ; because that other substance can 

 act by its mass, and can therefore overcome the affinity of 

 the carbonic acid, by acting successively : but to expel the 



