128 MOLECULAR MOTION AND ITS ENERGY 56 



constant volume. Hence the ratios for other gases could 

 easily be calculated from the known value of the ratio for 

 any one gas if their specific heats at constant volume were 

 known. 



But the specific heat at constant volume is with much 

 greater difficulty accessible to observation and measurement 

 than the specific heat at constant pressure. On this ground 

 no direct determinations of the former have been made. 1 

 But their values may be calculated from the measured values 

 of the latter by means of the law just given, which is 

 expressed by the last formula. Since this formula and law 

 are strictly exact only for perfect gases, the calculation can 

 indeed be admitted without hesitation only for such gases as 

 have no cohesion and strictly obey Boyle's law. For other 

 gases and vapours this calculation can only supply numbers 

 which, at most, can claim to be approximately correct esti- 

 mations. 



Still more doubtful becomes the interpretation of this 

 theoretical calculation if it is necessary to employ values of 

 the density which are not actually observed but' are theo- 

 retically deduced on the assumption of Boyle's and Gay - 

 Lussac's laws or of Avogadro's law. 



Finally, as a further cause that makes the values so 



calculated uncertain, must be added the circumstance ob- 



y^erved by Kegnault, 2 E. Wiedemann, 3 Winkelmann, 4 



* and Wiillner, 5 that the specific heat of many gases and 



vapours is variable in a high degree with the temperature. 



This is especially the case, according to an observation 



made by IVS^b-^J^ 6 with those compounds which contain 



carbon. 



With these reservations the values of the specific heat 



1 [Dr. Joly's direct determinations by means of his steam-calorimeter must 

 not be ignored. See Phil. Trans, clxxxii. 1891, p. 73 ; clxxxv. 1894, pp. 943, 

 961. TB.] 



2 M6m. de VAcad. de Paris, xxvi. 



3 Habilitationsschrift, Leipzig 1875 ; Pogg. Ann. clvii. 1876, p. 1. 



4 Pogg. Ann. clix. 1876, p. 177. 



5 Wied. Ann. iv. 1878, p. 321. 



6 Hohenheimer Programm, Stuttgart 1874, p. 82 ; Pogg. Ann. cliv. 1875, 

 p. 580. 



