426 PEINCIPLES OF CHEMISTKY 



identical composition. But this similarity of properties may either 

 proceed from the fact that one system of salts passes into the other (as 

 Bergman professed), in conformity with the predominating affinities 

 (for instance, from KCl + NaNO 3 there proceeds KX0 3 + NaCl, if it be 

 admitted that the affinity of the elements of the latter system be greater 

 than in the former) ; or, on the other hand, because both systems by 

 the interchange of a portion of their elements give one and the same 

 state of equilibrium, as according to Berthollet's teaching. Experiment 

 proves the latter. But before citing the most historically important 

 experiments verifying Berthollet's doctrine, we must stop to consider the 

 conception of the mass of the reacting substances. Berthollet by mass 

 understood the direct relative quantity of substances ; but now it is 

 impossible to understand this term otherwise than as the number of 

 molecules, for they act as chemical units, and in the special case of double 

 saline decompositions it is better to take the number of equivalents. 

 Thus in the reaction NaCl-fH 2 S0 4 , the salt is taken in one equivalent 

 and the H,SO 4 in two. If 2NaCl + H 2 S0 4 act, then the number of 

 equivalents are equal, and so on. The influence of mass on the measure 



of decomposition forms the root of Berthollet's doctrine, and there- 

 fore we will first of all turn our attention to the establishment of this 

 conception with regard to the double decomposition of salts. 



About 1840 H. Rose showed that water decomposes metallic sul- 

 phides like calcium sulphide, CaS, forming hydrogen sulphide, H 2 S, 

 notwithstanding the fact that the affinity of hydrogen sulphide, as an 

 acid, for lime, CaH 2 O 2 , as a base, causes them to react on each other, 

 forming calcium sulphide and water, CaS + 2H 2 O. Furthermore, Rose 

 showed that the greater the amount of water acting on the calcium 

 sulphide, the more complete is the decomposition. The results of this 

 reaction are evident from the fact that the hydrogen sulphide formed 

 may be expelled from the solution by heating, and that the resulting 

 lime is sparingly soluble in water. Rose clearly saw from this that 

 such feeble agents, in a chemical sense, as carbonic anhydride and 

 water, by acting in a mass and for long periods of time in nature on 

 the durable rocks, which resist the action of the most powerful acids, 

 are able to bring about chemical change to extract, for example, from 

 rocks the bases, lime, soda, potash. The influence of the mass of water 

 on antimonious chloride, bismuth nitrate, &c., is essentially of the same 

 character. These substances give up to the water a mass of acid which 

 is greater according as the mass of the water acting on them is greater.' 25 



25 Historically the influence of the mass of water was the first well-observed pheno- 

 menon in support of Berthollet's teaching, and it should not now be forgotten. In double 



