464 Intelligence and Miscellaneous Articles. 



dark space and the streaked positive jet, presenting, except the 

 streaks and the colour, an appearance analogous to the narrow 

 positive jet observed at about 8 or 10 millims. pressure. 



When, instead of a Greissler tube, we employed a large bell or one 

 of the balloons by aid of which the experiment was made of the 

 aurorae boreales with a central negative electrode encircled by the 

 positive ring, we still obtained the same effect ; that is, the large 

 spherical aureola, developed at very low pressures isolated around 

 the negative electrode, was replaced by a narrow blue jet of vivid 

 splendour, having sometimes the appearance of a brilliant blue flame 

 escaping from the positive electrode. This negative jet is always 

 produced in the continuation of the axis of the electromagnet, even 

 when the positive electrode is a ring situated in the same horizontal 

 plane as the negative electrode. The electricity, which escaped 

 equally in all directions from the negative electrode, now issues 

 only in one direction, as if projected to a distance from the mag- 

 netic pole. But it is only at very low pressures (1 millim., and 

 even lower) that the effect is produced with this degree of inten- 

 sity. The greater the elastic force of the gas, the shorter becomes 

 the negative dart, giving place to the positive jet. It is at about 

 2 millims. that the repulsion apparently exerted by the magnet 

 upon the negative aureola commences to become sensible. 



Such is the modification produced by magnetism in the appear- 

 ance of the electric discharge. It is accompanied by quite as marked 

 an alteration in the resistance opposed by the rarefied gas to the 

 passage of the discharge. As we had already observed, and recorded 

 in the memoir before cited, the magnet has the effect, in the case 

 of an axial discharge, of notably augmenting the intensity of the 

 current. 



With the above-described Greissler tube containing hydrogen, 

 placed vertically on the upper extremity of the soft-iron cylinder, 

 the negative electrode beneath the positive, the galvanometer placed 

 in the derived current marked 20° when there was no magnetization, 

 and 40° when the electromagnet was excited by 25 Bunsen couples. 

 The nitrogen-tube, placed in the same conditions, gave 20° without 

 magnetization, and 30° with. In another case, on throwing into 

 the electromagnet the current from 40 Bunsen couples, we saw the 

 deflection of the galvanometer increase, with the hydrogen-tube, 

 from 12° (before magnetization) to 55°, with the nitrogen-tube from 

 10° to 35°. These examples, taken at random from a great num- 

 ber of analogous results, show that the intensity of the discharge 

 transmitted through the Geissler tube may be quadrupled by the 

 action of an electromagnet sufficiently powerful*. They show, 

 moreover, what we have already recognized, that the effect on hy- 

 drogen is more marked than on air, that the augmentation of inten- 

 sity of the current is greater with the gas which is more conductive 

 than with that which is less conductive of electricity. 



* This augmentation of intensity is perceived by simple inspection of the 

 tube, from the fact that the negative electrode becomes red-hot and shows 

 traces of fusion as soon as magnetization commences. 



