419, 420] 



Numerical Values 



345 



way, assuming for (/A l) air the value 1 '000294. These ratios of values of 

 (fj, 1) are taken from Travers' Study of Gases, p. 296. The direct values 

 used are taken from Preston's Light, p. 137. 



The arrangement, it will be seen, is that of order of magnitude of (//. 1). 

 The order is also, with one slight exception, that of order of magnitude of \a. 

 The agreement of these two orders gives a sufficient indication of the general 

 accuracy of the values obtained for Jcr. 



The absolute values obtained are not so satisfactory, but this is hardly 

 surprising. For instance, from the value (//. l) air = '000294 we obtain the 

 absolute value 



Jo-=l'054x 10- 8 



of which the agreement with the value 



Jo- =1-41 x 10- 8 



could not be considered satisfactory if it were not for the large number of 

 inaccuracies which have necessarily been introduced in regarding the 

 molecules as elastic spheres. It should be noticed that this is the first 

 occasion on which we have been dealing with the actual matter of the 

 molecule and not with its field of force. 



Perhaps, then, we may see in the difference between the two values of Jo- 

 some indication of the extension of the field of force surrounding the 

 molecules. If so, regarding cr as variable with the temperature, the values 

 of o- calculated from //, 1 must be the limiting values of o- when T=<x>. 

 Looked at from this point of view, the value Jo-= T054 x 10~ 8 is in tolerably 

 good agreement with the figures tabulated on p. 256. 



