816 



Professor Sir James Dewar 



The relative rates of diffusion given in the table between the dif- 

 ferent gases is not preserved at all te'mperatures. Thus at 0' C. carbonic 

 acid has 16 times the rate of air, instead of 14 as given above for 15'_C. 

 In another case one film at 2' C. gave 1, 4 and l-Ga for the relative 

 rates of air, hydrogen and helium. Farther, the absolute nites, given 



-50 -40 -30 -20 -10 +10 +20 +30 +40 



Temperature °C. 



Fig. 17. 



in the second column, are mean values, the films differing slightly in 

 thickness. Extreme variations obtained were 1'2 and 3*2. 



The order of diffusibility shown in the above table is difficult to- 

 associate with any chemical or physical property of the gases con- 

 cerned. Especially is this the case with helium, the most volatile of 



