1859.] and its stratified Appearance in Rarefied Media. 9 



by the above experiments, that the identical vacuum tubes which show 

 the striae with certain modes of producing the discharge, do not show 

 them with other modes, and tliat therefore the striae are not a necessary 

 condition of the discharge itself in highly attenuated media, but 

 depend upon the mode of its production. 



The study of the electrical discharge in vacuo is of the utmost 

 importance in reference to the theories of electricity, and probably will 

 assist much towards the proper conception of other modes of force, 

 or, as they are termed, imponderables, heat, light, &c. 



The experiments of Walsh and Morgan, corroborated as they now 

 are by that of Mr. Gassiot, show, that although the transmission of 

 electricity across gaseous media is aided by rarefaction of the medium 

 up to a certain degree, yet that a degree of attenuation may be 

 reached at which the transmission ceases, at all events for a given dis- 

 tance between the terminals and given intensity of electrical charge. 

 Whether having arrived at this point a reduction of the space to be 

 traversed, or an increase of intensity in electricity, or both, would 

 again enable the electricity to pass, is not quite clear, though there is 

 reason to believe that it would, and the increased intensity of electricity 

 would probably be again stopped by a further improvement in the 

 vacuum, and so on. But the experiments go far to prove that ordinary 

 matter is requisite for the transmission of electricity, and that if space 

 could exist void of matter, then there would be no electricity : thus 

 supporting the views advocated by Mr. Grove and some others, that 

 electricity is an affection or mode of motion of ordinary matter. 



The non-transmission of electricity by very highly attenuated gas 

 may also afford much assistance to the theory of the aurora borealis, a 

 phenomenon, the appearance of which, the regions where it is seen, its 

 effect on the magnet, and other considerations, have led to the univer- 

 sal belief that it is electrical. 



The experimental result that a certain degree of attenuation of air 

 forms a good conductor, or easy path for the electrical force, while 

 either a greater or a less degree of density offers more resistance, and 

 this increasing towards either extremity of density or rarefaction, 

 show, that if there be currents of electricity circulating to or from the 

 polar regions of the earth, the return of which, as is generally believed, 

 gives rise to the beautiful phenomena of the aurora borealis or australis, 

 the height where this transit of electricity takes place would be just 

 that at which the density of the air is such as to render it the best con- 

 ductor. By careful measurement of the degree of attenuation requisite 

 to enable the electrical discharge to pass with the greatest facility in 

 our laboratory experiments, we may approximatively estimate the degree 

 of rarefaction of the atmosphere at the height where the aurora borealis 

 exists. By these means we get a mode of estimating the height of 

 the aurora by ascertaining, from the decrement of density in the atmo- 

 sphere in proportion to its distance from the earth, at what elevation 

 the best conducting state, or that similar to our best conducting 

 vacuum tubes would be found, or conversely, by ascertaining the 

 height of the aurora by parall-ictic measurements, we may ascertain 



