[ 205 ] 



XX VII. On the Specific Heat of Gases for Equal Volumes under 

 Constant Pressure. By Dr. Alexander Naumann*. 



§ 1. Ow the relative magnitudes of the several quantities which 

 make up the total specific heat of perfect gases, and on the con- 

 nexion between the total specific heat and chemical composition. 



THE uninterrupted development of the Mechanical Theory 

 of Heat has furnished continually increasing grounds of 

 support for Clausius's view, that the thermal content of a perfect 

 gas (that is, of one which obeys perfectly the laws of Mariotte 

 and Gay-Lussac) is represented by the progressive motion of the 

 molecules, and by the motions of the constituent atoms of the 

 molecules, In particular the principle maintained by Clausius, 

 according to which the vis viva of the progressive motion of the 

 molecules is, in the case of all gases, proportional to the abso- 

 lute temperature (counted from —273° C), has received a beau- 

 tiful experimental confirmation from the recent researches of 

 0. E. Meyer f upon the internal friction of gases. This view like- 

 wise receives unquestionable, though less direct, confirmation 

 from the immediate observation of similar molecular motions in 

 liquids J. 



When a perfect gas is heated under constant pressure, part of 

 the heat imparted to it is employed in effecting the external 

 work which accompanies the expansion of the gas : this portion 

 may be called the heat of expansion. Another part serves to in- 

 crease the vis viva of the progressive motion of the molecules : 

 this we may call the heat of molecular motion. A third part in- 

 creases the motion of the constituents of the molecules — that is, 

 of the atoms inside the molecules : this may be called the heat 

 of atomic motion. 



The heat of expansion was proved experimentally by Dulong§ 

 to be the same for all gases under the same pressure, and under 

 different pressures to be proportional to the pressure (though 

 the reason of it was first pointed out by Clausius||). For a 

 pressure of 760 millims. of mercury and an 'elevation of tempe- 

 rature from 0° to 1° C. (which are the conditions that will be 

 assumed in what follows when we have to deal with specific nu- 

 merical statements), this heat of expansion amounts to 00691^[ 



* Translated from the Annalen der Chemie und Pharmacie, vol. cxlii. 

 p. 265 (June 1867). 



t Pogg. Ann. vol. cxxv. passim ; vol. cxxvii. p. 253 (1865 and 1866). 



X Chr. Wiener, Pogg. Ann. vol. cxviii. pp. 85-91. 



§ Pogg. Ann. vol. xvi. p. 476. 



|| Pogg. Ann. vol. lxxix. p. 397 (1850). [Phil. Mag. S. 4. vol. ii. p. 21 ; 

 also ' Mechanical Theory of Heat' (Van Voorst, 1867), p. 42.] 



11 Ann. der Chem. und P harm. vol. cxviii. p. 1 16. 



