194 PHENOMENA DEPENDENT ON MOLECULAR PATHS 79 



given in 28, I calculate the density of argon at 0, referred 

 to that of water at 4 C., as 



p = 0-0017810, 



whence the mean speed at of the molecule of argon is 

 H = 381 metres per sec. 



For helium the numbers are somewhat uncertain; in the 

 mean Eamsay 1 found the value 2 -14 for the density when 

 that of oxygen was taken as 16. Hence follows, when the 

 density is referred to that of water, 



p = 0-000191, 

 for which the mean speed at is 



O = 1162 metres per sec. 



From these numbers we obtain then the molecular free 

 paths, 



for argon, 0-0000099 cm., 

 for helium, 0-0000240 



at and atmospheric pressure, and, finally, the collision - 

 frequency of a molecule 



for argon, 3840 x 10 6 per sec., 

 for helium, 4840 



The viscosity of mercury vapour has been measured by 

 S. Koch, 2 and the numbers found by him have later been 

 confirmed as correct by Noyes and Goodwin 3 by means 

 of comparative transpiration experiments. At the tempera- 

 ture 370 C. the value of the coefficient of viscosity was 

 found to be 



TJ = 0-000642, 



while according to the numbers given in 28 the value of 

 the molecular speed at this temperature must be taken as 



H = 260 metres per sec. 

 Hence from the formula 



= i7n?a/ 0-30967 p 



1 Proc. Roy. Soc. 1896, lix. p. 325 ; Zeitschr. f. phys. Chemie, 1896, xx. 

 p. 614. 2 Wied. Ann. 1883, xix. p. 857. 



3 Zeitschr. f. phys. Chemie, 1896, xxi. p. 671 ; Phys. Rev. 1896, iv. p. 207. 



