The Followers of Maxwell. 371 



will be remembered* that in the elastic-solid theory of 

 Boussinesq, the rotation of the plane of polarization of 

 saccharine solutions had been represented by substituting the 



equation 



e' = Ae + B curl e 



in place of the usual equation 



e' = Ae. 



Goldhammer now proposed to represent rotatory power in the 

 electromagnetic theory by substituting the equation 



E = (4ircVO D + k curl D, 

 in place of the customary equation 



E = (4ircVO D : 



the constant k being a measure of the natural rotatory power 

 of the substance concerned. The remaining equations are as 



usual. 



curl H = 47rD, - curl E = H 



Eliminating H and E, we have 



fi = (c 2 / ) V 2 D + (k/4w) V 2 curl D. 



For a plane wave which is propagated parallel to the axis of x, 

 this equation reduces to 



k_ 



47T 



k 



47T ~& ' 



and, as MacCullagh had shown in 1836,f these equations are 

 competent to represent the rotation of the plane of polarization. 

 In the closing years of the nineteenth century, the general 

 theory of aether and electricity assumed a new form. But 

 before discussing the memoirs in which the new conception was 

 unfolded, we shall consider the progress which had been made 

 since the middle of the century in the study of conduction in 

 liquid and gaseous media. 



*Cf. p. 186. tcf. p. 175. 



2B2 



