312 DIRECT PROPERTIES OF MOLECULES 113 



for different gases ; for this purpose the theoretical formula 

 for the coefficient of viscosity ( 76, 27), 



77 = wG/4 2 = 



or, more strictly by 78, 



77 = 0-30967 



is of service, in which, as before, m denotes the molecular 

 weight and fl the mean molecular speed. For two different 

 gases, which we distinguish by the subscripts 1 and 2, we 

 then obtain the ratio 



since for equality of temperature we have 



m^l^ = m 2 fl 2 2 or m^f^ = W 2 *n 2 . 



From this formula we see that the ratio of the sizes of the 

 spheres of action of two gases, viz. 



can be determined from the molecular weights and the 

 coefficients of viscosity. 



In order to be able to compare the values of the mole- 

 cular volume calculated by this formula with those given by 

 Kopp, the molecular volume V 2 for any normal gas, chosen 

 arbitrarily, with which the others are compared must be 

 put equal to the value found by Kopp. For this purpose 

 Lothar Meyer employed sulphurous acid, because its 

 specific volume seemed to be determined with greater 

 certainty than that of any of the other gases whose 

 viscosity had been accurately measured by Graham. In 

 this way he, and likewise Loschmidt, obtained values for 

 the molecular volume which in many cases agreed really 

 well with those calculated by Kopp from the density of 

 the liquid. 



But before I can tabulate the results I must mention 

 a striking circumstance which would be well suited to raise 

 objections against the accuracy of the calculation. Such 



