436 PRINCIPLES OF CHEMISTRY 



doctrine, we must stop to consider the conception of the mass-oi the 

 reacting substances. Berthollet understood by mass the actual rela- 

 tive quantity of a substance ; 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 it as the number of equivalents. Thus in the reaction 

 NaCl -f H 2 S0 4 the salt is taken in one equivalent and the acid in 

 two. If 2NaCl 4- H 2 S0 4 act, then the number of equivalents are equal, 

 and so on. The influence of mass' on the amount of decomposition 



-' forms the root of Berthollet's doctrine, and therefore we will first of 

 n 



all turn our attention to the establishment of this principle in relation 

 to the double decomposition of salts. 



About 1840 H. Hose M 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 2 , as a base, causes them to react on each othen 

 forming calcium sulphide and water, CaS -f 2H 2 0. 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 an timonious. chloride, bismuth nitrate, &c., is essentially of the same 

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

 is greater in proportion as the mass of the water acting on them is 

 greater. 2 * bis 



25 H. Rose is more especially known for his having carefully studied and perfected 

 several methods for the exact chemical analysis of many mineral substances. His pre- 

 decessor in this branch of research was Berzelius, and his successor Fresenius. 



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

 phenomenon in support of Berthollet's teaching, ana it should not now be forgotten. 

 In double decompositions taking place in dilute solutions where the mass of water ia 

 large, its influence, notwithstanding the weakness of affinities, must be great, according 

 to the very essence of Berthollet's doctrine. 



As explaining the action of the mass of water, the experiments of Pattison Muir (1879) 

 are very instructive. These experiments demonstrate that the decomposition of bismuth 

 chloride is the more complete the greater the relative quantity of water, and the less the 

 mass of hydrochloric acid forming one of the products of the reaction. 



