PEOFESSOE STOKES ON THE liONO SPECTETJM OE ELECTEIC LIGHT. 619 
of the spark, which, though necessary to start it, was formed at its expense ; and as in 
the arc discharge the jar was idle, the increase of resistance in a circuit already com- 
prising the secondary coil was unimportant. 
The fact that the blue negative light which appears when the arc discharge is formed 
shows air-lines, points to the air as the seat of the intense action which there takes 
place ; and the very high refrangibility of some of the rays emitted, and the copiousness 
of those rays, indicate how intense that action is. The heating of the negative electrode 
seems to be a secondary effect, not due to the direct passage of the electricity through 
the metal (for the section through which it passes is not by any means small), but to 
the heat communicated from the film of air investing it. Small as is the mass of the 
film compared with that of the portion of the electrode adjacent to it, the rate at which 
heat is communicated is enormous. Thus with a positive point nearly touching under- 
neath a negative electrode of platinum, foil containing water, the foil is kept red-hot 
under the water, though the mere passage of electricity through the metal would be quite 
inadequate to produce that effect. Corresponding to the heating of the electrode by the 
air is the cooling of the air by the electrode ; and such a powerful abstraction of heat 
can hardly take place without altering the state of the film of air in relation to its power 
of conducting electricity. This would seem to be the reason why the film of air in 
contact with the negative electrode behaves so differently from any arbitrary section of 
the column along which the discharge takes place, and from offering greater resistance 
becomes the seat of a more intense emission of highly refrangible rays. At the positive 
electrode, at which, for whatever reason, the issue of electricity is confined almost to a 
point, nothing of this kind takes place ; but, from the contraction of the section through 
which the electricity has to pass in the electrode, a minute portion of the metal of which 
it is composed is so highly acted on that matter belonging to the electrode is liable to 
appear in the arc. 
These views lead to curious speculations respecting the negative light in highly 
exhausted tubes, and respecting the remarkable reversion of heating-effect which Mr. 
Gassiot has obtained according as the discharge is intermittent or continuous*, but I 
forbear to speculate further, 
* Proceedings of the Eoyal Society, vol. xi. p. 329. 
