LIMITS OF INSULATION AND DISCHARGE IN AIR. 99 



in air to the pressure of the atmosphere. The latter is the view which, being adopted 

 by PoissoN and Biot*, is also, I believe, that generally received; and it relates two 

 such dissimilar things, as the ponderous air and the subtil and even hypothetical 

 fluid or fluids of electricity, by gross mechanical relations; by the bonds of mere static 

 pressure. My theory, on the contrary, sets out at once by connecting the electric 

 forces with the particles of matter ; it derives all its proofs, and even its origin in the 

 first instance, from experiment ; and then, without any further assumption, seems to 

 offer at once a full explanation of these and many other singular, peculiar, and, I 

 think, heretofore unrelated effects. 



1378. An important assisting experimental argument may here be adduced, derived 

 from the difference of specific inductive capacity of different dielectrics (1269. 1274. 

 1278.). Consider an insulated sphere electrified positively and placed in the centre 

 of another and larger sphere uninsulated, a uniform dielectric, as air, intervening. 

 The case is really that of my apparatus (1187.)5 and also, in eff'ect, that of any ball 

 electrified in a room and removed to some distance from irregularly formed conduct- 

 ors. Whilst things remain in this state the electricity is distributed (so to speak) 

 uniformly over the surface of the electrified sphere. But introduce such a dielectric 

 as sulphur or lac, into the space between the two conductors on one side only, or 

 opposite one part of the inner sphere, and immediately the electricity on the latter is 

 diff'used unequally (1229. 1270. 1309.), although the form of the conducting surfaces, 

 their distances, and the pressure of the atmosphere remain perfectly unchanged. 



1379. FusiNiERi took a different view from that of Poisson, Biot, and others, of 

 the reason why rarefaction of air caused easy diffusion of electricity. He considered 

 the eff'ect as due to the removal of the obstacle which the air presented to the expan- 

 sion of the substances from which the electricity passed-}-. But platina balls show 

 the phenomena in vacuo as well as volatile metals and other substances ; besides 

 which, when the rarefaction is very considerable, the electricity passes with scarcely 

 any resistance, and the production of no sensible heat ; so that I think Fusinieri's 

 view of the matter is likely to gain but few assents. 



1380. I have no need to remark upon the discharging or collecting power of flame 

 or hot air. I believe, with Harris, that the mere heat does nothing (1367.), the rare- 

 faction only being influential. The effect of rarefaction has been already considered 

 generally (1375.) ; and that caused by the heat of a burning light, with the pointed 

 form of the wick, and the carrying power of the carbonaceous particles which for the 

 time are associated with it, are fully sufficient to account for all the effbcts. 



1381. We have now arrived at the important question, how will the inductive 

 tension requisite for insulation and disruptive discharge be sustained in gases, which, 



* Ency. Britann. Supplement, vol. iv. Article Electricity, pp. 76. 81. &c. 

 t Bib. Univ. 1831. xlviii. 375. . 



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