CONSTITUTED OF SPHERICALLY SYMMETRICAL MOLECULES. 469 



tin- n;it mv r,t' th" .Ml'-iii-iti..!]. til-- gneoteoi i- extramdy good in '!! 



tin-re cases (of course the values of ^ for the pure gases are taken from the formula 

 34). The hypothesis that the molecules are rigid elastic spheres surrounded hy fields 

 of attractive force gives notably the best results, however, the agreement being 

 within 3 per cent, in every case which is little, if any, more than is introduced by 

 uncertainty of the data. 



The above are the only available data for monatomic gases, so far as I know. 

 WASSILEWJA* has determined the conductivity of mixtures of hydrogen and oxygen, 

 and, of course, we have the conductivity of air ; but as these are mixtures of 

 polyatomic gases our formula does not apply to them, so that they will not be 

 discussed here. 



In conclusion, it is hardly necessary to remark that our formula for the conductivity 

 of a mixed gas reduces to that for a pure gas on putting p or p' equal to zero. Smce 

 (as we shall see) D 13 , the coefficient of diffusion, varies inversely as the total pressure, 

 p u D 12 is independent of the pressure, as also are all the other quantities entering into 

 E, F, G, FI ; hence the coefficient of conduction of a mixture, like that of a pure gas, 

 is independent of the total pressure. This has been experimentally verified in the 

 case of air. t 



19. The Coefficient of Viscosity of Mixed (rases. 



The viscosity of mixed gases has been much studied, both theoretically and 

 experimentally. It is especially interesting on account of the curious fact, first 

 noticed by GRAHAM ('Phil. Trans.,' 1846), that the addition of a moderate amount of 

 light gas (hydrogen, in the case mentioned) to a much more viscous and heavy gas 

 (carbon dioxide) may actually increase the viscosity of the latter. The same 

 phenomenon was commented on by MAXWELL in a Bakerian Lecture (1866), the 

 gases receiving particular mention being air and hydrogen. 



The principal formulae which have been deduced for the viscosity of a mixture are 

 due to MAXWELL,]: PULUJ, SUTHERLAND, || and THIESEN.T Of these only the first is 

 based on an adequate proof, but as it is a particular case of my own formula (got by 

 putting k = A.J/A, = 0771) it does not need separate discussion. PULUJ'S formula is 

 based on an ingenious adaptation of the earlier theories of viscosity of a pure gas, and 

 although, like those theories, it is. only approximate, and in some ways not 



* 'Phys. Zeitschrift,' 5, 1904, p. 737. 



t MEYER, ' Kinetic Theory,' Chapter IX.; TODD, ' Roy. Soc. Proc.,' A, 83, p. 19, 1909. 

 I ' Scientific Papers,' vol. ii., p. 72. 



'CARb's Repertorium,' xv., p. 590; ' Wien. Sitzungsber.,' 1879, Ixxix., Abth. 2, pp. 97, 745; see also 

 MEYER, ' Kinetic Theory,' p. 200. 

 || 'Phil. Mag.,' 1895, xl, p. 421. 

 U 'Verb. d. Deutsch. Phys. Ges.,' 4, p. 238, 1902. 



