6 Professor Apjohn upon a new Method of 



1.421, in order that the representative of air should be unity. Column (4) ex- 

 hibits the specific heats under a constant pressure, as deduced by De la Roche and 

 Berard. 



The numbers in column (2) correspond so well with those in column (4), 

 which were experimentally obtained, that Dulong conceives himself entitled to 

 enunciate, as proved, the two following propositions, the first of which he has 

 assumed in his calculations. 



1st. That equal volumes of all gases, at the same temperature and pressure* 

 evolve by a given condensation the same amount of caloric. 



2nd. That the rise of temperature produced in each gas, by the heat so ex- 

 tricated, is reciprocally proportional to its specific heat, under a constant volume. 



At the close of his paper, which was read before the Academy of Sciences, in 

 May, 1828, M. Dulong states, that he was then engaged in researches, in refe- 

 rence to the influence of variations of pressure and temperature, on the specific 

 heats of gases, the results of which he hoped to be shortly able to give to the pub- 

 lic in a second memoir, in which it was his intention also to investigate the laws 

 which connect the specific heats of the compound gases with their actual compo- 

 sition. Eight years, however, have now elapsed since this promise was made, and 

 I am sorry to add, (indeed it will be considered matter of general regret,) that it 

 has not as yet been redeemed. 



The method of Dulong, just explained, is partly experimental, and partly 

 hypothetical. To the principle on which his experiments were conducted, no 

 possible objection can be urged, and I apprehend that the numbers at which he 

 arrived represent, with considerable precision, the ratios of the specific heats eJ" 

 the gases on which he operated, in the two different predicaments to which allu- 

 sion has so frequently been made. Doubts, however, may be entertained as to 

 the truth of his hypothetical premiss, that all gases, in virtue of a given compres- 

 sion, evolve the same amount of caloric ; and if this be incorrect, his conclu- 

 sions in reference to the specific heats of the gases under a constant volume, or a 

 constant pressure, must also be erroneous. 



These observations are thrown out, with the view of pointing attention to 

 what may be considered as proved, and what assumed, in the paper of Dulong ; 

 and of showing, that, even after his elaborate researches, the subject under consi- 

 deration must still be considered as constituting an open question. At all 



