14 



ELECTRICITY. 



the former exerts a repulsive action on 

 the fluid in the latter body ; and if this 

 happens to be a conductor, it impels a 

 .certain portion of that fluid to the remote 

 end of this body, which becomes at that 

 part positively electrified ; while its 

 nearer end, which the same fluid has 

 quitted, is consequently in the state of 

 negative electricity. If the first body 

 'had been negatively electrified, its unsa- 

 turated matter would have exerted an 

 'attractive force on the fluid in the 

 second body, and would have drawn it 

 nearer to itself, producing an accumu- 

 lation or redundance of fluid at the 

 adjacent end, and a corresponding defi- 

 ciency at the remote end: that is, the 

 former would have been rendered posi- 

 tive, and the latter negative. All this 

 is exactly conformable to observation. 



(54.) The phenomena of transference 

 are easily explicable on this hypothesis ; 

 and they arise from the destruction of 

 the equilibrium of forces, which con- 

 fined the fluid to a particular situation 

 or mode of distribution. 



(55.) There is, indeed, no fact ex- 

 plicable by the hypothesis of a double 

 fluid, which is not explained with 

 equal facility by that of a single 

 fluid, with the condition already stated. 

 The explanation by the first is easily 

 converted into an explanation by the 

 second, by substituting the expressions 

 of positive and negative for those of 

 vitreous and resinous electricities ; and 

 considering the action of the latter as 

 arising from the influence of redundant 

 or unsaturated matter, to which is 

 ascribed in the Franklinian hypothesis 

 a similar operation to that of the resi- 

 nous electricity in the hypothesis of Du 

 Fay. The hypothesis of a single fluid 

 has, it must be allowed, the advantage 

 of greater simplicity : but, on the other 

 hand, it lies open to the objection of its 

 involving a condition which appears, at 

 first view, to be at variance with our 

 preconceived notions of the primary 

 laws of matter, and more especially with 

 that of gravitation ; namely, that which 

 implies the mutual repulsion of its par- 

 ticles when void of electricity. 



When viewed as a mere hypothesis 

 calculated to facilitate our comprehen- 

 sion of the phenomena and of their con- 

 nexions, it is a matter of indifference 

 which we employ, for they will either of 

 them answer the purpose. In our fu- 

 ture explanations we shall, in general, 

 adhere to the language of the Frank- 

 linian theory, as being the simplest, and 



generally the most convenient ; and be- 

 cause a conversion of terms the reverse of 

 that just now pointed out will in all cases 

 enable us to supply the explanation of 

 the same phenomenon according to the 

 theory of Du Fay. As to the question 

 which of these two hypotheses ap- 

 proaches the nearest to the real state of 

 things, we are not yet prepared to dis- 

 cuss the arguments that could enable us 

 to decide it ; and we must, therefore, wait 

 till we can resume the subject in the 

 sequel. 



The further development of these 

 theories, and of the law of induction, 

 in particular, must, for the present, be 

 postponed, since they require us to be 

 acquainted with many practical details 

 relating to the accumulation of elec- 

 tricity, and its management when ap- 

 plied to various objects of experimental 

 research. 



CHAPTER III. 

 Electrical Machines. 



(5G.) THE essential parts of an instru- 

 ment for procuring large supplies of 

 electricity for the purposes of experi- 

 ment, or an electrical machine, as it is 

 called, are the electric, the rubber, the 

 prime conductor, the insulator, and the 

 machinery for setting the electric in 

 motion. 



(57.) The electric, by the excitation 

 of which the electricity is to be de- 

 veloped, may be made of various mate- 

 rials. Globes of sulphur were employed 

 by the earlier electricians for that pur- 

 pose ; but polished glass is found, on 

 the whole, to be the most convenient 

 substance. The original form given to 

 it by Hauksbee, who was the inventor 

 of the electrical machine, was that of a 

 globe, which he caused to revolve upon 

 a vertical axis. The most convenient 

 forms, however, are those of a hollow 

 cylinder, or of a flat circular plate, revolv- 

 ing upon a horizontal axis. When used 

 in the form of a globe or cylinder, it has 

 sometimes been found advantageous to 

 line the inside of it with a thin layer of 

 a resinous composition, consisting of 

 four parts of Venice turpentine, one of 

 resin, and one of bees' wax. This must 

 be introduced in sufficient quantity into 

 the inside of the globe or cylinder, and, 

 when the glass is brought gradually to 

 an equal degree of heat throughout the 

 melted substances, is allowed to spread 

 itself over the interior surface, by turn- 



