516 Mr. W. Sutherland on the Viscosity 



Naumann and L. Meyer (Lieb. Ann. Suppl. Bd. v. p. 253, and 

 Phil. Mag. 1867, xxxiv. p. 551) to the calculation of the 

 relative sizes of molecules, and with considerable success; 

 but when the same method was applied to vapours of liquids, 

 certain discrepancies arose which have caused this method of 

 inquiry as to molecular size to come to a standstill. ^ We can 

 now see that the reason for these discrepancies lies in the fact 

 that the form of relation for the viscosity of vapours is different 

 from that for gases, and also in the fact that with gases also 

 inaccuracies are introduced by ignoring molecular force in the 

 factor 1 + C/T by which it expresses itself. 



The published data from which values of C for other sub- 

 stances can be obtained are those of Obermayer (Sitz. Akad. 

 Wien, lxxiii.) on the variation of viscosity with temperature ; 

 the following values are calculated from his results. 







Values of 0. 





N 2 . 



0, 



CO. C 2 H 4 . 



N 2 0. 



84 



127 



100 272 



260 



The mean value of G for hydrogen from Obermayer's 

 experiments with different capillary tubes comes out 79, 

 ranging from 88 to 69, which is much smaller than the value 

 given by Barus's experiments, namely 113, the same value as 

 for air ; but it is to be remembered that the viscosity of 

 hydrogen is a difficult physical constant to measure on 

 account of the large effect of impurities, and moreover in 

 Barus's experiments at very high temperatures the hydrogen 

 began to pass through the walls of the platinum capillary tube, 

 and it is possible that a slight similar action at lower tempera- 

 tures might interfere with the apparent variation of viscosity 

 with temperature. As Barus's experiments were carried out 

 with a different object from that of getting the best value of 

 a constant for pure hydrogen, it is probable that Obermayer's 

 value, though derived from a temperature-interval of only 

 40°, is more nearly the value for pure hydrogen. For 

 nitrogen the value 84, obtained from Obermayer's experi- 

 ments, is too different from the value 113 for air to be quite 

 satisfactory ; so that I think it is better to derive the value for 

 nitrogen from those for air and oxygen, thus C 4/5 + 127/5 

 = 113, whence = 109. 



In the case of C 2 H 4 and JST 2 0, the values of the viscosity 

 found by Obermayer at —21° have been excluded in the 



calculation of C as coming from a region too near the 

 vaporous. 





