﻿the Combination of Acids and Bases, 37 



liquids. We have already seen an illustration of one of these 

 disturbing influences, in the fact that dilute nitric acid, when 

 mixed with water, gives a slight fall of temperature, hydrochloric 

 acid a rise ; and the differences of specific heat in the solutions 

 formed will, to a small extent, modify the results. But the 

 cause of the higher thermal power of sulphuric acid I have not 

 been able to discover ; and future researches must decide whether 

 it depends upon some disturbing cause, or (which is less probable) 

 upon its possessing an exceptionally high thermal power. One 

 condition, however, is essential, or Law 1 will not apply. The 

 acid and base must be capable of combining when brought into 

 contact, and of forming a stable compound. In the paper so 

 often referred to, I showed that hydrocyanic acid and potash, 

 which fail to fulfil this condition, do not disengage the normal 

 amount of heat when mixed; and the same observation will 

 doubtless be found to apply to a large number of metallic oxides, 

 which form unstable compounds with, and imperfectly neutra- 

 lize, the bases. 



As regards the experimental proofs of the other laws, even 

 those of the fourth law, the truth of which is admitted by MM. 

 Favre and Silbermann, they are only approximative ; and here 

 also we meet occasionally with peculiar and unexpected results. 

 Thus a slight fall of temperature occurs, as Hess showed long 

 ago, in the conversion of the neutral sulphate of potash into the 

 acid salt ; and I found, as indeed might have been expected from 

 their alkaline reaction, that in the conversion of the ordinary 

 phosphates and arseniates into super- salts, a disengagement 

 of heat occurs amounting to about one- seventh of that disen- 

 gaged in the formation of the salts themselves. In other cases 

 results, at first view startling and apparently anomalous, will be 

 found to be strictly in 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 acid 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 *. 



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



