252 Prof. De la Hive on the Propagation of Electricity 



the appearance of a very fine mist of a rosy white tint which ad- 

 vances into the tube, but which, when the introduction of the 

 additional quantity of gas has stopped, progresses from the ne- 

 gative to the positive electrode, allowing the dark space to be 

 reproduced, and hiding, as it passes, the successive stratifications 

 of the different parts of the column,, enveloping them as a light 

 cloud would do ; this mist afterwards disappears, and the lumi- 

 nous column resumes its first appearance, which it retains as 

 long as there is no change either in the flow of electricity or in 

 the state of the gas through which it passes. 



The appearance of this mist, which exactly resembles that 

 which I have already mentioned as existing within the dark 

 space of the column when in a state of rest, clearly indicates the 

 agitation which the introduction of a small additional quantity 

 of gas produces throughout the column, an agitation which is also 

 proved by the advance of the striae and their encroachment one 

 upon another. The phenomenon moreover exhibits this further 

 peculiarity — namety, that the sharpness and brilliance of the 

 striae in the additional portion of gas introduced, which distin- 

 guish them so clearly from those in the gas previously in the 

 tube, enable us to follow the movement of the additional gas 

 from one end of the tube to the other. The experiment may be 

 repeated several times in succession by the introduction of suc- 

 cessive portions of gas, provided that the pressure is not increased 

 by more than ^ a millim. each time, and that the total pressure 

 does not exceed more than 5 or 6 millims. in all. 



With nitrogen and atmospheric air the phenomena are simi- 

 lar ; but the experiment cannot be carried to the same extent, 

 as the pressure at which the effects cease to show themselves 

 is much low T er with these gases than it is with hydrogen. 

 The narrow striae which appear at the moment the gas .enters, 

 and at the end of the tube at which the entrance takes place, 

 are also less sharp and less brilliant ; but there is the same 

 momentary disappearance of the dark space, the same pro- 

 duction of a rose-coloured mist, and the same advance of this 

 mist, when the introduction of the gas has ceased, from the 

 negative to the positive electrode. With all three gases alike, 

 when the gas enters near the negative* electrode the mist is 

 seen advancing as a narrow vein along the axis of the tube from 

 the positive to the negative electrode; then, after arriving at this 

 end of the tube, it turns back and progresses, as we have said, 

 from the negative to the positive electrode. 



This mist is evidently caused by a portion of the gas which, 

 on entering the tube, becomes enormously expanded and is ren- 



* \Query, positive.] 



