XIX.] EXPERIMENT. 421 



image. Neither did the different matter of the prisms 

 make any : for in a vessel made of polished plates of glass 

 cemented together in the shape of a prism, and filled with 

 water, there is the like success of the experiment according 

 to the quantity of the refraction." But in the latter state- 

 ment, as I shall afterwards remark (p. 432), Newton 

 assumed an indifference which does not exist, and fell 

 into an unfortunate mistake. 



In the science of sound it is shown that the pitch of a 

 sound depends solely upon the number of impulses in a 

 second, and the material exciting those impulses is a matter 

 of indifference. Whatever fluid, air or water, gas or liquid, 

 be forced into the Siren, the sound produced is the same ; 

 and the material of which an organ-pipe is constructed 

 does not at all affect the pitch of its sound. In the science 

 of statical electricity it is an important principle that the 

 nature of the interior of a conducting body is a matter of 

 no importance. The electrical charge is confined to the 

 conducting surface, and the interior remains in a neutral 

 state. A hollow copper sphere takes exactly the same 

 charge as a solid sphere of the same metal. 



Some of Faraday's most elegant and successful researches 

 were devoted to the exclusion of conditions which previous 

 experimenters had thought essential for the production of 

 electrical phenomena. Davy asserted that no known fluids, 

 except such as contain water, could be made the medium 

 of connexion between the poles of a battery ; and some 

 chemists believed that water was an essential agent in 

 electro-chemical decomposition. Faraday gave abundant 

 experiments to show that other fluids allowed of elec- 

 trolysis, and he attributed the erroneous opinion to the very 

 general use of water as a solvent, and its presence in most 

 natural bodies.^ It was, in fact, upon the weakest kind of 

 negative evidence that the opinion had been founded. 



Many experimenters attributed peculiar powers to the 

 poles of a battery, likening them to magnets, svhich, by 

 tiieir attractive powers, tear apart the elements of a sub- 

 stance. By a beautiful series of experiments,^ Faraday 

 proved conclusively that, on the contrary, the substance of 



* Experimental Researches in Electricity, vol. i. pp. 133, 134. 

 ^ Ibid. voL i. pp. 127, 162, &c. 



