124 MOLECULAR MOTION AND ITS ENERGY 55 



volume, as, for instance, the molecular volume. Hence if, 

 as in 21* of the Mathematical Appendices, we denote the 

 mean energy of forward motion of a single molecule by E, 

 and the mean value of its internal atomic energy by (, the 

 ratio of these two magnitudes is equal to that calculated 

 above, viz. 



E K 



This somewhat altered conception allows us to compare 

 the mean amount of energy e possessed by a single atom 

 with the molecular energy. For if the number of atoms in 

 the molecule is n, then the desired mean value is 



-*. 



n 



Since air is not a chemical compound of unchangeable 

 composition it is not, strictly speaking, allowable to apply 

 this formula to it. But since its components, nitrogen and 

 oxygen, have the common property of possessing two atoms 

 in a molecule, we may also for air put n = 2, and obtain 



|=0-646, I = 0-323. 



The energy e of an atom is thus considerably smaller than 

 the energy E of progressive motion of a molecule of air. 

 This ratio is in agreement also with that for most other 

 gases, as the following table shows. 



The first column of figures contains the observed values 

 of the ratio of the specific heats for a series of gases and 

 vapours. The observations of Dulong (D) and Mas son 

 (Mn) are given according to Wiillner's corrected calcula- 

 tion ; also in Wiillner's (W) determinations the correc- 

 tions later applied byStrecker and Wullner are taken into 

 account. 1 In addition to these I have taken the observations 

 of Kontgen (E), P. A. Miiller (Mr), Strecker (S), 2 de 

 Lucchi (I/), 3 Martini (Mi), 4 Maneuvrier and Fournier 



1 Wiillner's Lehrbuch, 4. Aufl. 1885, iii. p. 522. 



2 Inaug. Diss. Strassburg ; Wied. Ann. xiii. 1881, p. 20 ; xvii. 1882, p. 85. 



3 Nuovo Cimento [3] xi. 1882, p. 11 ; Exner's Rep. xix. 1883, p. 249. 



4 Atti del 1st. Yen. [5] vii. 1880-1, p. 491; Landolt and Bernstein's 

 Tables, 2 ed. p. 340, tab. 137. 



