6 Mr, GrovCy on the Electrical Dischtirge, [Jan. 28, 



flame issues from a single nucleus, the wick ; and the amount of heat 

 produced is definite for a definite amount of chemical combination. 



In the Voltaic arc there are two points or foci; the polar terminals 

 there undergo a change, but not a consumption equivalent or nearly 

 so to the heat and light produced ; but if the consumption of the zinc 

 or the quantity of it combined with oxygen in the cells of the battery 

 be compared with the amount of heat generated in the arc, plus that in 

 the cells of the battery and conducting wires, the same amount of total 

 heat will be found to be developed as if the same quantity of zinc 

 were simply burned in oxygen. 



By subdividing more and more the plates of the voltaic battery and 

 proportionately increasing their number, we gradually increase the 

 length and diminish the volume of the arc, until at length we arrive, 

 as in the voltaic columns of De Luc and Zamboni, at the electric 



The spark from a Ruhmkorff coil was projected on a screen by the 

 electric lamp, and the impression contrasted with that of the flame of 

 a candle ; in the former two cones are seen to issue from the terminals 

 instead of the single one of the latter, one being more powerful, and 

 overcoming or beating back the other ; and this effect is reversed as 

 the direction of the current is reversed. 



In all cases hitherto observed there is a dispersion or projection of 

 a portion of the terminals ; this takes place in all forms of electric dis- 

 ruptive discharge, whatever be the materials of which the terminals 

 are composed. In the voltaic arc there is a transmission of matter, 

 principally from the positive, which is the more intensely heated, to 

 the negative terminal ; in the spark from the Ruhmkorff coil the dis- 

 persion is principally, and in some cases appears to be entirely, from 

 the negative terminal, while this is now the more intensely heated. 



In addition to this, there is generally, but not always, a change 

 produced in the medium across which the discharge passes ; compound 

 liquids, vapours, and gases are decomposed, and even elementary gases 

 are allotropically changed. There is also a polar condition of the 

 electrical discharge, which produces the converse chemical effects at 

 each pole — effects described by Mr. Grove in a paper in the Philo- 

 sophical Transactions for 1852, and subsequently shown at an evening 

 meeting of this Institution, 



Gases offer a powerful resistance to the passage of the discharge, 

 but this resistance is diminished as the gases are rarefied ; and a dis- 

 charge which would not pass across a space of half an inch in air of the 

 ordinary density will pass through several feet in highly attenuated air. 



In experimenting on the passage of the discharge through the 

 vapour of phosphorus in 1852, Mr. Grove observed for the first time 

 that the discharge was traversed by a number of dark bands or striae. 

 At first he was disposed to attribute this phenomenon to some peculi- 

 arity of the medium ; but on trying good vacua of other vapours and 

 gases, he found the striae were in all cases visible, and seemed to 

 depend on the degree of rarefaction of the gas. Many subsequent 

 experiments have been made by himself and others on the subject, and 



