580 A DYNAMICAL THEORY OF THE ELECTROMAGNETIC FIELD. 



is the number of electrostatic units in one electromagnetic unit of electricity, 

 and this, according to our result, should be equal to the velocity of light in 

 air or vacuum. 



The velocity of light in air, by M. Fizeau's * experiments, is 



7=314,858,000; 



according to the more accurate experiments of M. Foucaultt, 



F= 298,000,000. 



The velocity of light in the space surrounding the earth, deduced from 

 the coefficient of aberration and the received value of the radius of the earth's 

 orbit, is 



V= 308,000,000. 



(97) Hence the velocity of light deduced from experiment agrees sufficiently 

 well with the value of v deduced from the only set of experiments we as yet 

 possess. The value of v was determined by measuring the electromotive force 

 with which a condenser of known capacity was charged, and then discharging 

 the condenser through a galvanometer, so as to measure the quantity of electricity 

 in it in electromagnetic measure. The only use made of light in the experiment 

 was to see the instruments. The value of V found by M. Foucault was 

 obtained by determining the angle through which a revolving mirror turned, 

 while the light reflected from it went and returned along a measured course. 

 No use whatever was made of electricity or magnetism. 



The agreement of the results seems to shew that light and magnetism 

 are affections of the same substance, and that light is an electromagnetic dis- 

 turbance propagated through the field according to electromagnetic laws. 



(98) Let us now go back upon the equations in (94), in which the 

 quantities J and "V occur, to see whether any other kind of disturbance can 

 be propagated through the medium depending on these quantities which disappeared 

 from the final equations. 



* Comfits Rendiu, Vol. MIX. (1849), p. 90. 

 t Ibid. Vol. LV. (1862), pp. 501, 792. 



