SODIUM CHLORIDE BERTHOLLET'S LAAVS 431 



simple results. But much more instructive and complete are researches 

 similar to those made by Pattison Muir (1876), who took the simple 

 case of the precipitation of calcium carbonate, CaCO 3 , by the mixture of 

 solutions of calcium chloride and sodium or potassium carbonate, and 

 found in this case that not only was the rate of action (for example, in 

 the case of CaCl 2 + Na 2 CO 3 , 75 per cent, of CaCO 3 was precipitated 

 in five minutes, 85 per cent, in thirty minutes, and 94 in two 

 days) determined by the temperature, relative mass, and amount of 

 water (the mass of water decreases the rate and limit), but that the 

 limit of decomposition was also dependent on these influences. How- 

 ever, even in researches of this kind the conditions of reaction are 

 complicated by the non-uniformity of the media, inasmuch as a portion 

 of the substance is obtained or remains in the precipitate, so that the 

 system is heterogeneous. The investigation of double saline decompo- 

 sitions offers many difficulties, which cannot be considered as yet 

 entirely overcome. Many efforts have, however, long since been made. 

 In virtue of their historical interest, I will cite two of these efforts, 

 which are due to Thomsen (1869) and Ostwald (1876). 



Thomson applied a thermo-chemical method to exceedingly dilute 

 solutions without taking the water into further consideration. He 

 took solutions containing 100H. 2 O per NaHO, and sulphuric acid con- 

 taining J,H 2 SO 4 + 100H 2 O. If these solutions be mixed in such 

 proportions that atomic proportions of acid and alkali would act, then 

 for forty grams of caustic soda (which answers to its equivalent) 

 there should be employed 49 grams of sulphuric acid, and then 

 + 15689 heat units would be evolved. If the normal sodium sulphate 

 so formed be mixed with n equivalents of sulphuric acid, then a certain 



amount of heat is absorbed, namely a quantity equal to , - rrrrox heat 



units. An equivalent of caustic soda, in combining with an equivalent 

 of nitric acid, evolves +13617 units of heat, and the augmentation of 

 the amount of nitric acid entails an absorption of heat for each equiva- 

 lent equal to 27 units so also in combining with hydrochloric acids 

 + 13740 heat units are absorbed, and for each equivalent of hydro- 

 chloric acid beyond this amount there are absorbed 32 heat units. 

 Thus sulphuric acid evolves a somewhat greater quantity of heat than 

 nitric and hydrochloric acids ; the difference being approximately equal 

 to 2000 heat units per equivalent of caustic soda. From this it might 

 be concluded that nitric acid or hydrochloric acid would not act on 

 sodium sulphate. In reality, Thomsen found this not to be the case. 

 Nitric and hydrochloric acids decompose sodium sulphate to a much 

 more considerable extent than sulphuric acid decomposes sodium 



