1890.] Specific Heats of Gases at Constant Volume. 441 



varied. It is found that the specific heats of these gases are not 

 constant, but are variable with the density. In the case of air the 

 departure from constancy is small and positive ; that is, the specific 

 heat increases with increase of the density. The experiments afford 

 directly the mean value 0'1721 for the specific heat of air at the 

 absolute density of 0'0205, corresponding to the pressure of 19'51 

 atmospheres. A formula based on the variation of the specific heat 

 with density observed in the experiments ascribes the value O1715 

 for the specific heat at the pressure of one atmosphere. The formula 

 assumes the specific heat to be a linear function of the density, which 

 must as yet be regarded only as an approximation, the exact nature 

 of the relation being concealed by variations aaiong the experiments. 



These results appear to be in harmony with the experiments of 

 Wiedemann on the specific heat at constant pressure, and of Rowland 

 on the mechanical equivalent of heat, from which the value 0'1712 is 

 deduced for C at 760 mm. 



The experiments on carbon dioxide reveal a more rapid variation of 

 the specific heat with density, the variation iu this case being again 

 positive in sign. The formula 



C, = />x 0-2064 +0-16577 



appears with considerable reliability to express the relation between 

 specific heat and density. 



The relation between specific heat and density in the case of 

 hydrogen is of a negative character; that is, the specific heat 

 diminishes with increase of density. The experiments are chiefly 

 directed to elucidate this point, for, owing to the difficulty of pre- 

 paring pure hydrogen, it was found that variations in the quantita- 

 tive results of experiments on different samples of the gas were 

 unavoidable. Accordingly the experiments were directed to a com- 

 parison of the specific heats of like samples of the gas at different 

 densities. The variation with density is small, but (with one excep- 

 tion) all experiments on the purer hydrogen ascribe a negative 

 character to it. 



The nature of these variations of specific heat with change of 

 density is, in the case of the three gases, in accord with their beha- 

 viour as regards Boyle's law, within the limits of pressure. 



The experiments were effected in the steam calorimeter, a differential 

 method being used in which an empty or idle vessel is thermally com- 

 pared with the vessel holding the gas at high pressure. The vessels 

 possessing approximately the same calorific capacity, the result, 

 theoretically, is as if the gas was dealt with isolated from any con- 

 taining vessel. Although practically this is not attained, many sources 

 of error are eliminated by the procedure adopted. 



