392 



RECENT PROGRESS IN PHYSICS. 



in both cases at the angular distance of 30°. Thus, after the division, 

 the fiee electricity of the inner coating was nearly equal in the two 

 instruments, and as might have been predicted, was half as great on 

 each as on I before the division. 



In apparatus II, half of the air was now 

 replaced by another dielectrical medium, 

 {Faraday thus names the medium through 

 which electrical induction takes place). Shel- 

 lac was first tried. The upper half of the 

 apparatus II was removed, in the under part 

 of the sphere A, a hemispherical cup of 

 shellac was placed, and the upper half re- 

 turned again to its place, so that the inter- 

 vening space between the lower half of the 

 two spheres was filled, as shown in fig. 36. 



Apparatus I, which remained unchanged 

 as in the first experiment, was charged again, 

 in the same manner as before, and the free 

 electricity of the inner coating, measured by 

 the torsion balance. Thus, the result was by 



Apparatus 1 290°. 



The charge was now divided between I and 

 II, and after the division the result was. 



Apparatus I 114°, 



_ Apparatus II 113°. 



Here, also, the free electricity of the interior coating of the two in- 

 struments is very nearly equal after the division, but it is far less than 

 the half of the free electricity of apparatus I before the division ; hence, 

 it follows, that apparatus II had received more than lialf the electricity 

 of I, but without the free electricity on II being more than on I, and 

 consequently Faraday concludes that a more powerful induction takes 

 place through shellac. If we represent the quantity of free electricity 

 of the interior coating of I, before the division, by 290°, then the 

 whole quantity of its electricity will be n 200 ; after the division 

 there remained only w 114; hence there has been given to apparatus 



n (290 — 114) zr n 176. 



In Faraday's opinion another relation takes place between the latent 

 and I'ree electricity ; the free electricity is 113, the latent is w'. 113 ; 

 we have, consequently, 



n! 113 = n ],!76, 

 hence 



n! ■= n II !] ■=. n 1.55. 



Accordingly, an inductive force 1.5§ times greater takes place through 

 shellac than through air ; or, as Faraday expresses it, shellac has 1.55 

 times greater specific inductive capacity than air. 



By similar experiments Faraday found the specific inductive capa- 

 city of sulphur to be 2.24 times as great as that of air. 



For the various gases Faraday found their inductive capacity equal 

 to that of air. In order to introduce the difi'erent gases into the ap- 



