Dispersion in Relation to the Electron Theory, 457 



Here v is the total number of valencies in the molecule 

 and p is the number of electrons calculated with e/m = 

 4'5 x 10 17 e.s.u., which is somewhat smaller than the normal 

 value. 



In the above no attempt is made to distinguish between 

 the resonators which influence the refraction but have no 

 measurable effect on the dispersion and those to which the 

 dispersion is due. This of course can only be done when 

 from an extended series of measurements of n the constants 

 a have been calculated. There are a few substances for 

 which these constants have been determined. The results for 

 these, recalculated for the normal value of e/m(5*325 X 10 17 ), 

 are given below. 



Table II. 



Substance. 



v. 



Pi- 



1 1 ! 



Substance. v. p v 



Quartz 



8(16) 3-94 

 4 (16) 3-49 



Water ' 4 (8) "88 



Fluorite 



Carbon bisulphide ... 8(16) 1'93 



Benzene 30 3"35 



Naphthalene bromide. 48 (54) \ 3"06 

 Methyl iodide 8(14) 2"90 



Sylrine 



2 (8) 



2 (8) 



12 (24) 



1-92 

 3-54 

 4-42 



Rock-salt 



Calc spar (ord.),.. 





As X-! in the formulae is a mean value, the above numbers 

 give the upper limit to the number of electrons of longest 

 ultraviolet free period, except in so far as the unknown a alue 

 of e\m may affect the result. The values of pe\m used for 

 the first seven substances are those calculated by Drude, and 

 for the last three those of Erfie. The refraction measure- 

 ments of Erfle were confined to the visible spectrum, 

 but carried to a high degree of accuracy, and the constants 

 obtained do not differ notably from those given in Part I. 

 of this paper. 



Drude concludes that " the number of electrons in the 

 molecule which influence the dispersion is equal to or less 

 than the sum of the valencies of the atoms." 



( B) JSumber of Electrons deduced from Magnetic Rotation. — 

 It has been shown in Part I. that the only electrons in- 

 fluencing the magnetic rotation are those whose periods are 

 sufficiently long to affect the dispersion, and the mag- 

 netic property therefore provides an alternative method of 

 estimating the number of electrons of this type. Before 

 proceeding to deduce the formula we must give certain 

 secondary effects some further consideration. 



