investigating the Specijic Heats of the Gases. 3 



The most interesting, if not the most recent inquiries in refei'ence to the 

 specific heats of the gases, have been conducted by M. Dulong of Paris. In his 

 memoir upon the subject, published in the Annales De Chimie, (tom. xli. p. 1 13,) 

 this distinguished philosopher commences with a critique upon the processes of 

 Haycraft, and of Marcet and De la Rive, having for its object to show that the 

 law of " equal specific heats under equal volumes,"* at which they had arrived, 

 however strongly recommended by its simplicity, cannot be considered as un- 

 equivocally established by their experiments. It is difficult, indeed, to urge any 

 valid objection against the method of Haycraft. In prlru;iple it was the same 

 with that previously practised by De la Roche and Berard, and he, in addition, 

 took the very proper precaution of operating upon gases deprived of all hygrome- 

 tric moisture. M. Dulong, however, observes, and with truth, that apparently 

 unimportant variations in the manner of conducting the experiments would 

 greatly influence the results, and that Mr. Haycraft has not furnished sufficient 

 details to enable his readers to judge of the amount or direction of the errors by 

 which they were likely to be affi^cted. 



The objections to the researches of Marcet and De la Rive are of a much 

 graver natui-e. These philosophers included the different gases successively in 

 the same globe of glass, and having determined experimentally the times that 

 each thus enclosed took, when exposed to a constant heat, to acquire the same 

 rise of temperature, these times were concluded to be proportional to their 

 specific heats. To obtain, however, the times of heating of equal volumes of the 

 different gases, it is obvious, that we must subtract from those given by experi- 

 ment the times in which the glass balloon — supposed perfectly exhausted — would 

 undergo the same change of temperature. But owing to the insignificant amount 

 of the mass of the gas compared to that of its envelope, this difference will neces- 

 sarily always be so small, as to be, in all probability, frequently exceeded by the 

 inevitable errors of observation. Besides, as Dulong observes, and as was shown 

 by him and Petit in their celebrated prize essay on the Laws of Cooling in diffe- 

 rent Elastic Media, the times of heating of the different gases in the experiments 

 of Marcet and De la Rive, depended not exclusively upon their respective specific 

 heats, but also greatly upon their specific gravities. For these reasons M. Du- 



• They are, of course, all supposed to be submitted to the same pressure. 



B 2 



