On the Dimensions of Atoms. 517 



the author's measurements of the viscosities previously 

 recorded in various papers*, and represent the distances o£ 

 nearest approach of the centres of two atoms during a 

 collision, the Sutherland correction having been applied. 

 These distances it has been customary hitherto to call the 

 diameters of the atoms. In order to facilitate comparison 

 the increments of diameter in passing from one element to 

 the next have been placed in columns 3 and 5. Adl the 

 dimensions are given in Angstrom units (A = ]0 _s cm.). 



Table I. 



Element. 



From Crystal measurements;. From Viscosity measurements. 



Atomic 



diameter. 



Increment. 



Atomic 

 diameter. 



Increment. 





1-30 -J 



- 



2-3.3 j 

 •2-70 j 



075 



0-30 

 0-35 



2-02 | 

 2-56 | 

 2-76 j 

 3 06 



0-54 

 020 

 0-30 



Argon 



Krypton .... 

 Xenon 



With regard to these figures, it should be borne in mind 

 that whereas those in the second column represent the actual 

 diameter of the outer electron shell, those in the fourth cannot 

 be regarded as such. It is quite improbable that even in 

 a direct collision between thermally agitated atoms, the 

 approach is so close that the electrons of the atoms intermingle. 

 The mean distance between atomic centres during collisions 

 may therefore be expected to be in excess of the diameter of 

 the outer electron shell of each participating atom. 



The above figures support this view, all the numbers in 

 column -4 being in excess of the corresponding numbers in 

 column 2, although the order of magnitude is the same. More- 

 over, although there is no evidence of strict proportionality, 

 which, quite possibly, we are not entitled to expect, there is 

 general accordance between the increments of diameter in 

 passing from one electron shell to the next. Thus, according 

 to both estimates, there is a comparatively large increment in 

 passing from neon to argon, followed by one less than half as 

 large in the step between argon and krypton, and, finally, 

 a somewhat larger one between krypton and xenon. This 

 is fairly good agreement, considering the wide difference 

 between the methods of estimation. 



3. The halogen gases. — In these cases the comparison 

 * A. O. Rankine, Phil. Mag. vol. xxii. p. 45, and vol. x.vix. p. 552. 



