242 MR. HARRIS ON SOME ELEMENTARY LAWS OF ELECTRICITY. 



particles interposed between the points, from and toward which, the accumulated 

 electricity tends to flow. It is hence extremely doubtful, whether a general distri- 

 bution of electricity in mere space would ever occur, supposing the electrified sub- 

 stance to be the only existing body in the universe : directly, however, that we assume 

 the existence of another body, then in a space devoid of resistance, the resulting in- 

 duction would generate an attractive force, which, however small, would cause an 

 electrical current to flow through a distance, however great. 



75. In accordance with this deduction, it may be shown, that an electrified sphere 

 having an extremely perfect insulation, and projecting within the centre of a very 

 large receiver, retains its electricity more completely under a diminished pressure, 

 than in the atmosphere ; especially under ordinary conditions of aerial currents, im- 

 perfect insulation arising from extraneous bodies, and the like. This fact seems to 

 have hitherto escaped detection, and therefore, the notice it merits ; and I am inclined 

 to believe, that we may eventually find it requisite to modify, to some extent, our views 

 of the cause of electrical dissipation. The following experiment is of singular interest 

 as bearing upon this point. 



(m;.) a small brass sphere of about two inches in diameter Z», fig. 11, was placed 

 immediately in the centre of a very large globular receiver, by means of a brass rod 

 projecting into the receiver, and cemented airtight by an appropriate flanche of 

 brass and sealing-wax. The exterior extremity of the rod was connected with a de- 

 licate electroscope, and the sphere charged with a given quantity of electricity. 

 Under these circumstances the air was gradually withdrawn from the receiver, but 

 no sensible collapse of the electroscope had occurred when ^-l-ths of the air was with- 

 drawn. 



"i^. Common electricity traverses with greater or less facility, under an adequate 

 attractive force ^ the surface of any substance relieved from the pressure of a non-con- 

 ducting medium. If a glass rod, or a rod of wood, be passed through a tall receiver, 

 and be opposed to a point projecting from the conducting plate covering its upper 

 extremity, then on exhausting the air and continuing to electrify the insulated plate, 

 we shall eventually perceive electrical streams flowing over the rod ; and if we sub- 

 stitute a small wire for the rod, the same thing happens, except that the streams do 

 not usually appear in the surrounding glass, presenting in each very beautiful phe- 

 nomena. 



77. Discharges of common electricity are transmitted in this way more readily on 

 the surface of bodies, in an exhausted medium, than voltaic currents, the latter re- 

 quiring but little comparative insulation : it is difficult to fuse a fine wire in an ex- 

 tremely exhausted receiver by ordinary electricity, whilst voltaic electricity will soon 

 heat it to redness. I have discharged upwards of twenty-five square feet of coated 

 glass upon a fine wire of iron, inclosed in a well exhausted receiver, without in the 

 least affecting it ; the redundant electricity appeared to find an easier passage through 

 the rarefied air on its surface, producing an extremely brilliant effect ; whereas on ad- 



