AND MODERN PHYS1CH. 207 



By the time tho first edition of tho " Electricity 

 and Magnetism" was published, Maxwell and Thomson 

 (Lord Kelvin) had both made determinations of K, 

 and had shewn that for air at least tho resulting value 

 for the velocity of electro-magnetic waves was very 

 nearly that of light 



For other substances at that date the observations 

 were fewer still Gibson and Barclay had determined 

 tho specific inductive capacity of paraffin, and found 

 that its square root was 1*405, while its refractive index 

 for long waves is 1*422. Maxwell himself thought 

 that if a similar agreement could be shewn to hold 

 for a number of substances, we should be warranted 

 in concluding that " the square root of K, though it 

 may not be the complete expression for the index of 

 refraction, is at least tho most important term in it." 



Between this time and Maxwell's death enough 

 had been done to more than justify this statement. 

 It was clear from the observations of Boltzmann, 

 Silow, Hopkinson, and others that there were many 

 substances for which the square root of the specific 

 inductive capacity was very nearly indeed equal to 

 the refractive index, and good reason had been given 

 why in some cases there should be a considerable 

 difference between the two. 



Hopkinson found that in the case of glass tho 

 differences were very large, and they have since been 

 found to be considerable for most solids examined, 

 with tho exception of paraffin and sulphur. For 

 petroleum oil, benzine, toluene, carbon-bisulphide, and 

 some other liquids tho agreement between Maxwell's 

 theory and experiment is close. For the fatty oils, 



