50 Dr. A. 0. Rankine on the Relation hetween 



Now, making use o£ Sutherland's equation 

 /T \*/C + T.\ 



as applied to the emanation we obtain 



•-""O'Q-** 



= 2-130 xlO- 4 . 



This is the estimated value of the viscosity of radium 

 emanation at 0° C. It is recorded by means of the cross on 

 fig. 1. It lies between the values for xenon and krypton, 

 and its position strongly suggests that the up and down, 

 distribution of the points is not without significance. The 

 alternations were already regular but of decreasing ampli- 

 tude before the addition of the point representing the 

 emanation ; its addition carries the rule a step further in 

 both respect*. Of course, it is possible that another un- 

 identified member of this group of gases lies between xenon 

 and radium emanation ; but it will be seen that a place could 

 be found for it on the diagram. Indeed, a point higher than 

 X or Em and lower than Kr, placed about midway between 

 X and Em, would be more in keeping with the general trend 

 of distribution than a single step from X to Em. 



It is now possible to compare the molecular dimensions o£ 

 emanation and helium, since the viscosities at the same 

 temperature are known. The connexion according to the 

 kinetic theory is 



jSEm _ %e. /^Em 

 *He ^Em PrJ 



where s is the radius of the molecule and p the density of 

 the gas under constant conditions. This gives 



SEm _ 



2-57, 



^Em _ 

 VRe 



16-97, 



and consequently 



where v denotes the molecular volume. 



Thus the molecule of emanation is larger than that of any 

 other known gas in the group, as will be seen from the 

 following table. The other figures are copied from a 



