82 Prof. H. L. Callendar on the Tliermodynamical 



the value of the index w = 2, as in the Joule-Thomson equa- 

 tion. There can be no doubt, however, from experimental 

 ■evidence, that the energy of flight may be less than half 

 the total kinetic energy of a polyatomic molecule, otherwise 

 the ratio S/s of the specific heats could not be less than 4/3. 

 It is probable that the distribution of energy in the molecule 

 depends on the type or form of the molecule, and not merely 

 on the number of atoms it contains, and that the various 

 degrees of freedom are not all of equal value. The ratio of 

 the energy of rotation E /; to the energy of flight W in the 

 case of C0 2 is about 4/3, corresponding to the ratio of specific 

 heats S/s = 9/7. Whence, if n=2, the ratio E"/E' should be 

 7/3 for a coaggregated pair of molecules. For steam, which 

 is also a triatomic molecule, the loss of energy on coaggrega- 

 tion is greater. We have ra = 3'3 = s/R, so that the whole 

 energy of a coaggregated pair is no greater than that of a 

 single molecule. It is further possible that the relative 

 importance of the different kinds of degrees of freedom in a 

 complicated molecule would vary with the temperature. We 

 could not then assume that the limiting value S of the 

 specific heat at zero pressure was constant. The assumption 

 8 = constant is almost certainly true for monatomic or dia- 

 tomic molecules at ordinary temperatures ; but it could not 

 be true for unstable molecules, and there is some evidence 

 that it does not hold for polyatomic molecules of higher 

 orders. 



18. Application to Monatomic Gases. 



The only observations so far available to test the hypothesis 

 n = l/2 in the case of monatomic gases, are those of Ramsay 

 and Travers (Phil. Trans. A, 190 L) on the compressibility of 

 the inert gases by the capillary-tube method at 11 0, 2 C. from 

 20 to 80 metres pressure, and at 237°'3 C. from 30 to 80 

 metres. These observations are not very suitable for the 

 purpose, as they do not extend to low pressures. They also 

 exhibit, as the authors point out, several anomalies, which 

 may be due to some hitherto unexplained peculiarities in the 

 behaviour of monatomic gases, or perhaps merely to experi- 

 mental errors. The curves representing the variations of pv 

 with p at the lower temperature are of a perfectly normal 

 type, the gases helium, neon, argon, krypton, and xenon, 

 following naturally in the order of their densities. It should 

 be remarked, however, that if we produce the curves for argon 

 (39*9 gm.) and krypton (81*5 gm.) to zero pressure, they 

 appear to indicate a limiting value of pv equal to 18,500 



