122 ANNUAL OF SCIENTIFIC DISCOVERT. 



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 por- 

 tion of the terminals; this takes place in all forms of electric disruptive 

 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 dispersion 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, vapors, 

 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 Philosophical Transactions for 1832. 



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

 resistance is diminished as the gases are rarefied; and a discharge Avhicli 

 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 vapor of 

 phosphorus in 1852, Mr. Grove observed, for the first time, that the discharge 

 was traversed by a number of dark bands, or stria?. At first he was disposed 

 to attribute this phenomenon to some peculiarity of the medium ; but on 

 trying good vacua of other vapors and gases, he found the stria? 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 more particularly by Mr. Gassiot; and the extent of knowledge 

 we have acquired upon this still mysterious phenomena was now discussed 

 and illustrated. 



In the vapor of phosphorus, the strife generally exhibit themselves like 

 narrow ruled lines, about 0.05 inch diameter, transverse to the line of dis- 

 charge; but with certain precautions they become wider, and assume a coni- 

 cal form, somewhat resembling the whalebone snakes made as a toy for 

 children. Mr. Gassiot has used most carefully prepared Torricellian vacua, 

 and has also, in conjunction with Dr. Frankland, obtained excellent vacua, 

 by filling tubes containing sticks of caustic potass with carbonic acid, ex- 

 hausting them by the air-pump, and allowing the residual gas to be absorbed 

 by the carbonic acid. 



The following is a summary of the effects produced by the electric dis- 

 charge through these vacua : If the vacuum be equal to that generally ob- 

 tained by an ordinary air-pump, no stratifications are perceptible; a diffused 

 lambent light fills the tube; in a tube in which the rarefaction is carried a 

 step further, nai-row stria? are perceptible, like those first described in the 

 phosphorus vapor experiment. A step further in rarefaction increases the 

 breadth of the bands ; next we get the conical, or cup-shaped form ; and 

 then, the rarefaction being still higher, we get a series of luminous cylinders 

 of an inch or so in depth, with narrow divisions between them. Lastly, with 



