416 Mr. S. Russ on the Diffusion of 



abscissae represent distances along the plate and the ordinate^ 

 the logarithm o£ the corresponding activity. For convenience 

 in representing the results the initial ordinate has always 

 been taken through the same point. The mean of three 

 values of the coefficient for air was -096, which is somewdiat 

 lower than '112, the value found by Debierne. In the light 

 of subsequent work with different diffusion vessels, which 

 gave higher values, it is thought that the rather low value 

 obtained is due to the fact that the preparation of actinium 

 did not completely cover the base of the diffusion vessel, this 

 would cause some of the emanation to diffuse to the sides of 

 the vessel, and consequently make the vertical gradient too 

 steep. This, however, does not interfere with a comparison 

 being made in the different gases. 



According to Graham's law the diffusion coefficient of a 

 gas is inversely proportional to the square root of the mole- 

 cular weight of the gas into which it is diffusing. Hence, 

 taking the value obtained for the diffusion coefficient of the 

 emanation into air, we may calculate its coefficient when 

 diffusing into the other gases which have been worked with. 

 This has been done and the calculated values given at the 

 bottom of Table I. (p. 414). 



It will be seen that the differences between the experi- 

 mental and calculated values get larger as we go from a 

 heavy gas to a light one; but that even for hydrogen there 

 is agreement to within about 10 per cent., so that one may 

 say that if discrepancies do exist in the diffusion processes,, 

 they are exhibited fairly equally by these different gases. 



Actinium Emanation in Argon. 



As already stated, it was thought advisable to observe the 

 behaviour of the emanation when diffusing into a monatomic 

 gas ; for this purpose argon was chosen. With the kind 

 assistance of Mr. J. N. Pring about 400 cubic centimetres of 

 the gas were prepared, by passing air, first over red hot 

 copper to remove the oxygen, and then over calcium, heated 

 strongly in an iron tube, to absorb the nitrogen. After 

 passing the gas repeatedly over the calcium a density deter- 

 mination gave the value 19'7, which indicated a purity 

 sufficient for the purposes of the experiment. (Density of 

 argon = 20.) 



Owing to the small quantity of gas available smaller 

 cylindrical diffusion vessels were made; one with two parallel 

 glass plates suspended above some actinium, spread so as to 

 completely cover the base of the vessel, and the other fitted 



