CONTRIBUTIONS TO SCIENCE. 547 



carbons, and the like, does not differ sensibly from that of a 

 vacuum, and hence in these substances the velocity of light 

 must be inversely proportional to the square root of their 

 specific inductive capacity ; or, since the index of refraction 

 of a medium is the ratio of the velocity of light in a vacuum 

 to its velocity in that medium, it follows that the refractive 

 index must be directly proportional to the square root of the 

 specific inductive capacity. As all our measurements of 

 specific inductive capacity refer to the action of electro- 

 motive forces which continue for a much longer time than 

 the duration of a luminous vibration, we should expect the 

 last mentioned relation to agree most nearly with experiment 

 the longer the wave length of the light, or, as it is some- 

 times stated, the specific inductive capacity of a dielectric 

 should be equal to the square of its refractive index for 

 " light of infinite wave length" 



The results of the measurements of the specific inductive 

 capacity of certain liquids by Silow, and of gases, sulphur, 

 paraffin, and resin, agree with this theory as well as can be 

 expected. Boltzmann also finds that the specific inductive 

 capacities of crystalline sulphur along its three crystallo- 

 graphic axes are different, these differences coinciding with 

 tfre differences of the squares of the refractive indices for 

 light transmitted along these three directions. 



Dr. Hopkinson (Phil. Trans. Part II. 1881) has recently 

 measured the specific inductive capacities of turpentine, 

 benzol, petroleum, ozokerit lubricating oil, castor oil, sperm 

 oil, olive oil, and neats' foot oil. The hydrocarbons give 

 results which are quite in accordance with Maxwell's 

 theory, but the fatty oils, which are compounds of glycerine 

 with fatty acids, have inductive capacities far too great. The 

 same appears to be the case with all the varieties of glass 

 tested by Hopkinson, the specific inductive capacities of which 

 vary from 6 "61 in the case of very light flint to 9 '8 9 6 for 

 " double extra-dense " flint. In the case of solid paraffin, 

 Hopkinson's result agrees very nearly with that of Boltzmann 

 and with Maxwell's theory. In the case of glass, as in that 

 of the fatty oils, the high specific inductive capacity is 



