Energy of Chemical Actions. 279 



me, therefore, quite explicable by the theory of energy, without 

 the help of any hypothesis concerning the distances of the 

 molecules. 



4. Kirchhoff discusses the heat of combination in his memoir 

 on the effective function, and demonstrates that in general it must 

 alter with the temperature*. This follows also as a direct con- 

 sequence of the following considerations. Let explosive gas be 

 converted into water at different temperatures, e. g. at 50° and 

 100° C, in a closed space, and therefore without development 

 of external work ; the heat of combination can be the same in 

 both cases only if the difference between the quantities of energy 

 contained in the water and in the explosive gas at the two tem- 

 peratures is the same. This again implies that aqueous vapour 

 and explosive gas take up equal quantities of heat when they 

 undergo equal elevations of temperature — in other words, that 

 under constant volume the specific heat of water-vapour is the 

 same as that of explosive gas. This cannot in general be 

 assumed. 



Kirchhoff finds for this difference between the specific heats 

 of water- vapour and explosive gas, upon two distinct hypotheses, 

 0-0417 and 0*212 unit of heat per 1° C. We find similarly, 

 in the case of the formation of carbonic acid from carbonic oxide 

 and oxygen, 0*0049 unit per degree and per gramme. 



Since the alteration of the specific heat with the temperature 

 has been determined for only a few substances, this difference 

 cannot yet, in most cases, be exactly stated. 



5. The heat of combination is in general regarded as a mea- 

 sure of chemical affinity; but although experiment shows on 

 the whole a greater heat of combination in the case of stronger 

 combinations, many striking exceptions nevertheless present 

 themselves which make it impossible to accept this as a fully 

 proved principle; Thus phosphoric acid has a greater heat of 

 combination than sulphuric acid, although the latter displaces 

 it from its compounds. Potash is a stronger base than lime, 

 but evolves less heat than lime does by union with nitric acid. 

 Oxide of silver neutralizes the properties of acids more com- 



* Poggendorff's Annalen, vol, ciii. p. 203. [This had been distinctly 

 implied some years previously by Thomsen (of Copenhagen) in his Thermo- 

 chemical Investigations (Pogg. Ann. vol. lxxxviii. p. 349,1853, No. 3). It is 

 perhaps also allowable to refer to Watts's f Dictionary of Chemistry/ vol. iii. 

 p. 1.17, where the probability is pointed out that the thermal effect of a given 

 chemical change is not absolutely constant, but is affected by the circum- 

 stances under which the change takes place, and where (before the publi- 

 cation of the present memoir, and in ignorance of what had been said by 

 Kirchhoff) the writer deduces the almost necessary variation of the heat 

 of chemical action with the temperature, from considerations regarding 

 specific heat essentially the same as those insisted on by Kirchhoff (loc. cit.) 

 and by the author in the text. — Transl.] 



