79 



VISCOSITY OF GASES 



197 



The periodicity is exhibited most clearly in the first 

 group, which embraces the diatomic molecules ; it is repeated, 

 however, in the same way, but less markedly, in the second 

 group, which contains the triatomic molecules. In both 

 groups as the molecular weight rises, the coefficient of 

 viscosity so alters as to first of all rise, and then fall after 

 having attained a maximum. The free path diminishes in 

 both groups at first, then undergoes a slight increase, and 

 finally diminishes again. Lastly, the collision-frequency 

 falls off at first to a minimum, and then increases. 



Whether these relations hold good with monatomic 

 molecules can scarcely be judged from the numbers at 

 hand. To this class of gases belong not only mercury 

 vapour, but also argon and helium, because both these 

 gases, in common with mercury, have this property, that 

 for them the ratio of the two specific heats has the value l 

 1-67 ( 54). We should then have to tabulate the following 

 numbers calculated for C. : 



Of these the values of the coefficient of viscosity and 

 the collision-frequency exhibit the behaviour described, but 

 not the values of the free path. 2 We must not, however, 

 forget that the credibility of the foregoing figures is open to 

 sundry objections, and especially because the variation of 

 the viscosity of mercury vapour with the temperature has 

 not been determined with sufficient certainty. On the other 

 hand, doubts have been often expressed whether argon and 

 helium are really simple substances, and not, perhaps, 

 mixtures, and whether the molecule of argon has really only 

 one atom, and not perhaps two or three. In the first two 



1 Proc. Boy. Soc. 1895 ; Zeitschr. f. phys. Chemie, 1895, xvi. p. 363 ; Journ. 

 Chem. Soc. 1895, Ixvii. p. 684 ; Wied. Beibl. xix. p. 674. 



2 [The values for three gases only cannot show more than is shown 

 above. TR.] 



