248 Dr. Andrews on the Heat developed, Sfc. 



conversion of the neutral phosphates and arsenlates Into basic salts. In reality, 

 an equivalent of water is here again replaced by an equivalent of alcali, just as 

 occurs in the direct combinations of the acids and alcalis.* 



* When the experiments detailed in the foregoing paper were almost completed, I received 

 the 6th No. oiPoggendorff's Annalen, for 1840, containing the first part of a valuable Memoir, by 

 M. Hess, entitled " Thermo-chemical Researches." The experiments detailed by M. Hess refer 

 principally to the heat developed when sulphuric acid and water combine together — a subject not 

 touched upon in the present paper. He has, however, extended his inquiry to the heat evolved 

 during the combination of sulphuric acid with potash, soda, ammonia, and lime ; and also of hydro- 

 chloric acid with potash, soda, and ammonia. But the results obtained by M. Hess cannot be im- 

 mediately compared with those given in this communication, as his experiments were performed 

 with stronger acids, which disengaged heat when diluted with water. The quantity of heat thus 

 extricated, M. Hess has shown to be the same, whether the acid and water be mixed together in 

 presence of a base or alone ; and he has likewise furnished accurate data, by means of which the 

 heat derived from this source, in his experiments, may be estimated. Now, assuming with him, 

 as a term of comparison, the number of grammes of water which would be heated through 1° cen- 

 tigrade, by saturating with each alcali 1 gramme of sulphuric acid, or the corresponding equivalent 

 (0.908 gr.) of hydrochloric acid — all taken in the state of very dilute solutions — we deduce from the 

 foregoing tables the numerical results in the first of the following columns ; while those in the second 

 are derived from the memoir of M. Hess : 



TABLES. HESS 



rPotash, 407 . . .406 



Sulphuric Acid with . . -s Soda, 



■ 



Ammonia, 



C Potash, 

 Hydrochloric Acid with < Soda, 



V. Ammonia, 



413 . . . 411 



352 . . . 403 



364 .. . 362 



373 . . . 368 



310 . . . 318 



It is very satisfactory to observe how closely these numbers agree with each other, with the 

 single exception of that which expresses the heat evolved when sulphuric acid and ammonia com- 

 bine. The cause of this discrepancy I have endeavoured in vain to discover ; but it probably depends 

 upon some condition in the experiment of M. Hess, which may have escaped my observation. 



