360 Mr. J. C. Maxwell on Mr. Grove's 



That N=-^ is also variable with the temperature, appears 



distinctly from the second Table, even if the observations above 

 60° are not taken into account, since small errors in the index of 

 refraction exercise a great influence upon the value of B. 



The hitherto advanced empirical relations therefore cannot be 

 maintained, and the so-called " specific refractive force " (speci- 

 fische brechende Kraft) changes with the temperature. 



I consider it unnecessary to endeavour to introduce new rela- 

 tions between the velocity of the propagation of light and the 

 density of bodies, since mathematical investigations will certainly 

 soon give the desired solution, and then it will merely remain to 

 compare the theory with experiment. 



The proof that Schraufs " specific force " M [specif sche Kraft) 

 and his " specific dispersion power " N (specifisches Dispersions- 

 vermbgen), which has been given by the adduced numbers, are 

 not constants, overthrows all the conclusions grounded upon this 

 assumption, and the further investigations based upon them. 

 The differential equations which he gives are accordingly not 

 justified by experience. 



XLII. On Mr. Grove's " Experiment in Magneto-electric Indue- 

 Hon." In a Letter to W. R. Grove, F.R.8.* 



8 Palace Gardens Terrace, W. 

 Dear Sir, March 27, 1868. 



SINCE our conversation yesterday on your experiment on 

 magneto-electric induction f, I have considered it mathema- 

 tically, and now send you the result. I have left out of the 

 question the secondary coil, as the peculiar effect you observed 

 depends essentially on the strength of the current in the primary 

 coil, and the secondary sparks merely indicate a strong alterna- 

 ting primary current. The phenomenon depends on the mag- 

 neto-electric machine, the electromagnet, and the condenser. 



The machine produces in the primary wire an alternating 

 electromagnetic force, which we may compare to a mechanical 

 force alternately pushing and pulling at a body. 



The resistance of the primary wire we may compare to the 

 effect of a viscous fluid in which the body is made to move back- 

 wards and forwards. 



The electromagnetic coil, on account of its self-induction, re- 

 sists the starting and stopping of the current, just as the mass 



* Communicated by Mr. W. R. Grove, F.R.S. 

 t See Phil. Mag. S. 4. March 1868, p. 184. 



