246 Dr. Andrews on the Heat developed 



resembling, in this and its other thermal properties, the ordinary acids. Denoting 

 the pyrophosphoric acid by Pyr. we have, 



(NaO + ^PyrA)+iPyrA 

 (NaO+|PyrA) + iPyr.A 



55. The formation of the alcaline subphosphates and subarseniates, by the 

 direct union of the common phosphates and arseniates, with an additional 

 equivalent of base, is accompanied with a definite evolution of heat. On adding 

 to solutions of these salts, containing the equivalents of alcali before referred to 

 (NaO, .234 gr. KO, .353 gr.), alcaline solutions having half as much base as was 

 already in the salts themselves, I obtained very uniform results. 



56. That the heat produced was connected with the formation of the sub- 

 salt, appears distinctly from the circumstance, that a further addition of alcali 

 was not attended with any increase of temperature. The absence of any heat in 

 the case of the pyrophosphate of soda is easily explained on the same principle, 

 as Graham has shown that no subpyrophosphate of soda exists. 



57. The formation of these subsalts exercises a remarkable influence on the 

 quantities of heat developed, when the base is neutralized by successive portions 

 of acid. In ordinary cases, the heat evolved in this way is proportional to the 

 quantity of acid added. Thus, on mixing a solution of pure potash with one- 

 fourth, one-half, &c., an equivalent of nitric acid, the elevations of temperature 

 will be one-half, one-fourth, &c. of what is observed when the alcali is completely 

 neutralized. And the same principle I find to hold good, when successive por- 

 tions of the phosphoric (tribasic) and arsenic acids are added to solutions of the 

 pure alcalis, till the subsalts are formed ; but, after that point, a very different 

 law is followed, as will be seen in the next tables : 



