COMBINATION OF ACIDS AND BASES. 93 



about -|th above the mean, and the tartaric acid group a deviation of about -g^th 

 below it ; the bases, on the contrary (and the subsequent researches of Favre and 

 Silbermann have confirmed this result), differ altogether in thermal power from 

 one another. Thus equivalents of the oxides of magnesium and of silver give out 

 4 0, 1 and 1 0, 8 of heat respectively in combining with nitric acid, the former oxide 

 having therefore 23 times the thermal power of the latter. Yet, as is well 

 known, both these bases fully saturate the acid, and the resulting solutions are 

 even neutral to test paper. For these reasons, I have no doubt whatever that 

 the first law, as enunciated in 1841, is the expression of a true physical law, 

 and that in the combination of acids and bases in presence of water the heat 

 disengaged is determined by the base and not by the acid. It is true that in 

 this, as in similar physical inquiries, experimental results cannot immediately 

 be obtained free from complication or disturbing influences. The same remark 

 applies to the experimental proof of the great law discovered by Dulong and 

 Petit, which connects the specific heats and atomic weights of the elementary 

 bodies, and also to that of the remarkable relations discovered by Kopp between 

 the composition and boiling points of many organic 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, hydro- 

 chloric 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 is, however, 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 hydro- 

 cyanic 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 neutralise, 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 disengaged 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 



VOL. XXVI. PART I. 2B 



