IS* On the Electrical Phcenomena exhibited in Vacuo. 



charged, the spark passed through nearly as much space as 

 in common air, and with a hght visible in the shade. At all 

 temperatures below 200", the mercurial vacuum was a much 

 worse conductor than highly rarefied air: and when the 

 tube containing it was included in the exhausted receiver, its 

 temperature being about 50", the spark passed through a di- 

 stance six times greater in the Boylean than in the mercurial 

 vacuum. 



It is evident from these general results that the light (and 

 probably the heat) generated in electrical discharges depends 

 priiicipallij on some properties or substances belonging to the 

 ponderable matter through which it passes; but they prove 

 likewise that space, where there is no appreciable quantity of 

 this matter, is capable of exhibiting electrical phsenomena; 

 and, under this point of view, they are favourable to the idea 

 of the phaenomena of electricity being produced by a highly 

 subtile fluid or fluids, of which the particles are repulsive, 

 with respect to each other, and attractive of the particles of 

 other matter. On such an abstruse question, however, there 

 can be no demonstrative evidence. It may be assimied, as in 

 tlie hypothesis of Hooke, Huygens, and Euler, that an ethereal 

 matter, susceptible of electrical aflPections, fills all space; or 

 that the positive and negative electrical states may increase 

 the force of vapour from the substances in which they exist; 

 and there is a fact in favour of this last idea which I have often 

 witnessed — when the Voltaic discharge is made in the Boylean 

 vacuum, either from platinum or charcoal, in contact with 

 mercury, the discharging surfaces require to be brought very 

 near in the first instance; but the electricity may be after- 

 wards made to pass to considerable distances through the va- 

 pour generated from the mercury or charcoal by its agency; 

 and when two surfaces of highly fixed metal, such as platinum 

 or iron, are used, the discharge will pass only through a very 

 small distance, and cannot be permanently kept up. 



The circumstance, that the intensity of the electrical light 

 in the mercurial vacuum diminishes as it is cooled to a certain 

 point, when the v.ipour must be of almost infinitely small den- 

 sity, and is then stationary, seems strongly opposed to the idea, 

 that it is owing to any permanent vapour emitted constantly 

 by the mercury. The results with tin must be regarded as 

 more equivocal ; because as this substance cannot be boiled 

 in vacuo, it may be always suspected to have emitted a small 

 quantity of the rare air or gas to which it has been exposed : 

 yet, supposing this circumstance, such gas must be at least as 

 highly expanded as the vapour from cooled mercury, and can 

 hardly be supposed capable of affording the dense light, wliich 



the 



