638 Mr. W. Sutherland on the 



iodine at about 1 100° C. is about the same as that of glass at 

 300° C, and enormously less than that of ordinary electrolytic 

 solutions. But the vapour-density measurements of Crafts 

 and Meier show that at this temperature about a quarter 

 of the I 2 is dissociated into 21, and therefore if Richarz's 

 simplifying hypothesis were true for iodine the conductivity 

 ought to have been found enormously greater by J. J. Thomson 

 than it was. Until this difficulty with I 2 and the corre- 

 sponding one with N 2 4 has been cleared up, we do not know 

 what becomes of the electric charges which Richarz assumes 

 to be separated, and are therefore not entitled to write e 2 /r 

 or e 2 /s as the electrical work done in dissociating a molecule 

 such as I 2 or N 2 4 . To account for the facts with iodine we 

 shall have to assume a rearrangement of doublets rather than 

 the splitting of them into separate electrons. Moreover, we 

 have in Helmholtz's theory to take account of changes in 

 the mutual potential energy of electrons and atoms in any 

 change of relative positions caused by rearrangement of 

 doublets. Therefore at present we must not attach too 

 much importance to the agreement in order of magnitude 

 between our s and Richarz's r as derived from Helmholtz's 

 theory in a form which is the simplest possible and is appa- 

 rently too simple for the facts of nature. There is obviously 

 room for immediate further experimental and theoretical 

 work in this interesting department. 



4. Period of Rotation of an Electric Doublet. 



If we carry out the Richarz simplification to one of its 

 logical consequences we shall consider the two electrons of a 

 molecule like NaCl, when giving the line spectrum of Na, to 

 be revolving round one another with their relative path 

 (perlaps entirely in the Na atom) a circle of such size that 

 centrifugal force and electric attraction are in equilibrium. 

 Let i be the inertia of an electron, then 



iv 2 _e 2 1 _ e 2 



where K is dielectric constant and N index of refraction for 

 NaCl (preferably perhaps for Na), and for the period of 

 revolution we have 



2irs 2ttNA'* 



with the values s of order 4 x 10~ 9 , i/e = 345 x 10" 17 -r-500 

 (J. J. Thomson), e of order 3 x 10~ 10 , and N=1'5, this gives 



