224 Theory of a Non-Conservative Gas [CH. x 



and 7 have very approximately these values in the case of air, of the elements 

 hydrogen, nitrogen and oxygen, and of the gases CO, NO and HC1. These 

 molecules are all diatomic, so that the structure which our theory indicates is 

 fully in agreement with what we should expect. A molecule symmetrical 

 about an axis as regards both shape and structure would obviously be 

 formed by the conjunction of two spherical atoms. It might also conceivably 

 be formed by two atoms which were themselves not spherical but each 

 symmetrical about an axis, the axes coinciding with that of the molecule 

 as a whole. The original shape of the atoms, whether spherical or not, will 

 possibly be lost when the atoms are placed under the influence of each other's 

 forces, but there is nothing to destroy the symmetry about the single axis of 

 figure of the whole molecule. 



More complex Gases. 



271. The next value possible is n + 3 = 6, giving 7 = 1. This is 

 obviously the value which ought to be found for a gas in which the molecules 

 may be regarded as rigid bodies, having no axis of symmetry. We might 

 reasonably have expected all triatomic molecules and molecules of higher 

 atomicity to be of this type. 



It is, however, quite clear from experiment that for many gases the 

 value of 7 is very much lower than 1^. In other words, the molecules of 

 many substances must not be regarded simply as rigid bodies, for the 

 energy of internal motion is comparable with the energies of translation 

 and rotation. 



272. Various attempts have been made to connect the values of n and 7 

 with the number of atoms in the molecule. Naumann*, for instance, 

 suggested that n is identical with the number of atoms in the molecule, 

 while J. J. Thomson j- suggests that in the special case of a symmetrically 

 arranged molecule, n + 3 may be found to be proportional to the number 

 of atoms in the molecule. 



The experiments of CapstickJ have, however, shewn quite conclusively 

 that no general law can be expected to relate 7 with the number of atoms, 

 independently of the nature of the atoms. For instance, he finds the follow- 

 ing values for the methane derivatives : 



7 n + 3 



Methane CH 4 1-313 6'4 



Methyl Chloride CH 3 C1 1-279 7'2 



Methylene Chloride CH 2 C1 2 T219 9'0 



Chloroform CHC1 8 1154 13'0 



Carbon tetrachloride CC1 4 1130 154 



* Annalen der Chemie, cxtin. (1867) p. 284. 



t Watts 1 Dictionary of Chemistry, i. p. 89. 



J Phil. Trans. CLXXXVI. (1895) p. 564 ; CLXXXV. (1894) p. 1. 



