94 DR ANDREWS ON THE HEAT DEVELOPED IN THE 



accordance with the general principles already laid down. In the formation of 

 double salts there is no disengagement of heat — a principle announced in 

 1841, and which ought perhaps to be enunciated as a distinct law, although it 

 is implicitly involved in Law 2. Again, if tribasic phosphoric acid or arsenic 

 acid is added in fractional portions to a solution of potash till the subsalts are 

 formed, the heat disengaged on each addition of acid corresponds to the amount 

 of acid added ; but after this point has been reached, the disengagement of heat 

 follows a different law. The pyrophosphoric acid, on the other hand, behaves 

 in the same way as the nitric and most other acids, when added in successive 

 portions to solutions of potash or soda; equal increments of heat being evolved 

 for equal additions of acid, till the pyrophosphate of potash or soda is formed.'- 



APPENDIX. 



In the following tables I have given the results described in this communi- 

 cation and those of 1841 in a form which admits of comparison with one 

 another, and with those of MM. Favre and Silbermann. I have also added a 

 few determinations recently made by M. Thomsen of Copenhagen, t It will be 

 seen that the original experiments of 1841 exhibit, on the whole, a fair agree- 

 ment with those now communicated to the Society. From the small scale on 

 which they were performed (the whole weight of the solutions after mixture 

 being less than 30 grammes), the imperfect form of the apparatus, and the 

 uncertainty of the thermometric indications, I have indeed been surprised to 

 find them so near the truth. The results of MM. Favre and Silbermann do 

 not exhibit the precision which might have been expected from the high char- 

 acter of those experimentalists, and from the accuracy of other parts of their 

 great work. The mercurial calorimeter employed by them appears to have 

 been little adapted to its purpose ; but after making due allowance for its im- 

 perfections, I am at a loss to account for the serious errors into which they have 

 fallen. M. Thomsen's experiments have evidently been made with care, and 

 his results agree comparatively with my own ; but the absolute amount of heat 

 obtained by him falls far short of what I have found. It is indeed much easier 

 to obtain results relatively than absolutely correct. The numbers given in this 

 paper will, I believe, be found rarely to differ relatively more than ^oth from 

 the truth, but they may hereafter require a small correction in respect to their 

 absolute value. That correction can, however, be scarcely more than -^th of 

 the whole amount ; and I have little doubt that the number, for example, 



* Transactions of the Royal Irish Academy, vol. xix. pp. 245-248. The observations of Graham 

 confirm the statement that no heat is evolved in the formation of any double salt. Memoirs of the 

 Chemical Society, vol. i. p. 83. 



t Poggendorff's Annalen, cxxxviii. p. 78. 



