2 Professor Apjohn upon a new Method of 



masses. But a given weight of air, in cooling through any number of degrees, 

 will evolve different quantities of caloric, according to the circumstances under 

 which its refrigeration is effected. If it be permitted to shrink as it cools, so that 

 its elasticity may continue constant, it will obviously extricate more heat than if 

 its primitive volume be maintained by being enclosed in, for example, some un- 

 yielding envelope ; inasmuch as experiment proves that after a gas has cooled 

 down in the latter predicament, a considerable rise of temperature takes place, 

 when upon admitting the atmosphere it is subjected to its original pressure. The 

 specific heat of a gas, therefore, it should be borne in mind, admits of a double 

 interpretation, or is different according as the gas is considered to be of a constant 

 volume or of a constant elasticity. 



Now, of the many philosophers who have applied themselves to researches in 

 reference to the specific heats of the agriform fluids, some (as Crawford, Clement, 

 and Desormes, Marcet and De la Rive,) have experimented upon the gases 

 maintained at a constant volume; while others, (as Lavoisier, Laplace, Gay 

 Lussac, Leslie, De la Roche, and Berard, and finally Haycraft,) upon the same 

 at a constant elasticity ; so that for this reason, even if there were no other, their 

 experimental results, and the numerical conclusions thence deduced, do not all 

 admit of immediate comparison. 



But though we collate those results alone which are deduced by the same 

 method, a great discordancy will be found to exist between them. The method 

 of Cawford, Clement, and Dcsormes, and of Marcet and De la Rive, were in 

 principle the same, as all operated on the gases preserved of a constant volume; 

 and, nevertheless, the conclusions at which they have arrived are widely different. 

 Nor is there a closer agreement between the numbers arrived at by those who 

 have essayed the solution of the problem, by determining the quantities of caloric, 

 evolved by the different gases in cooling, under a constant pressure, through the 

 same range of temperature. Some, as Leslie and Haycraft, have arrived at th^ 

 law, since so ably advocated by Marcet and De la Rive, that all gases have, 

 under equal volumes, the same capacity for caloric, or what amounts to the same 

 thing, that the spex;ific heats of equal weights are reciprocally proportional to their 

 specific gravities ; while others, as Lavoisier and Laplace, Gay Lussac, and, in 

 particular, De la Roche and Berard, have obtained results quite irreconcilable 

 with so simple a view of the subject. 



