388 Extension of Maxwell's Electro-magnetic Theory of Light. 



electrical resistances. Hence if 7 in the present instance might be 

 identified with the molecular viscosity as determined by Mr. Tom- 

 linson, a connection similar to that derived experimentally by Knndt 

 would be established. Calculating by the above approximate 

 process the real parb of the refractive index from Drude's experi- 

 mental data, a fair agreement, as to order of magnitude, is found 

 with the same quantity when calculated accurately. The agreement 

 is not, however, close enough to explain the very accurate propor- 

 tionality between the velocity of light in a metal and the conduc- 

 tivity of the latter which Kundt's figures imply. Since, however, 

 Pfliiger has shown that the temperature co-efficients of the velocities 

 of light and the conductivities are not of the same order of magni- 

 tude, the process employed above may perhaps represent the nature 

 of the physical connection between the two quantities to a sufficient 

 degree of approximation. 



11. [Added May 10. Another relation of some importance can be 

 readily obtained from (17). It is a well-established experimental 

 law, often made the basis of exact chemical determinations, that the 

 co- efficient of absorption of a solution of an absorbent substance 

 in a transparent liquid is proportional to the number of molecules 

 of the absorbent present in unit volume of the solution. A simple 

 extension of the reasoning formerly used will give for the square of 

 the refraction index of such a mixture the value 



Cll , C&Z 

 .-1 



TO" 27rmr 



(18), 



where Ci n x , T\ are constants for the transparent medium, whilst 

 c 2 , w 2 , T 2 , w, and 7 refer to properties previously defined of the 

 dissolved absorbent substance. Further we may write 



ft = A + m 2 B, 



where A, in dilute solutions, is nearly independent of the amount of 

 colouring matter present, whilst 



27T7?IT 



4?r 2 w T 



for a substance possessing only one absorption band, and is to a first 

 approximation independent of the properties of the solvent. 



Then /*~ = R 2 (cos 2z + 1 sin 2z) = A + m 2 B. 



