176 PHENOMENA ACCOMPANYING 



ou the part of ucj^ative electricity, a great facility in dispersing itself in the 

 ambient medium, Avhen once that medium is rarefied. 



The {i^:;itation of the stria? in the luminous part of the current becomes very 

 considerable under the i?light pressure of two millimetres. It manifests itself 

 at iirst very sensibly in the neij^hborhood of the jiositive electrode, from which 

 the luminous stream issues under the form of an outspreading cone, which, in 

 proportion as the pressure diminishes, becomes more and more cylindrical, until 

 it assumc-s altogether the form of a cylinder of whose circular base the electrode 

 is the centre, the agitation of the striae being, at the same time, general through- 

 out the whole extent of the current. • 



When the discharge is effected in a cylindrical jar, between a ball serving as 

 a negative electrode and a metallic ring of which that ball is the centre, and 

 which serves as a positive electrode, the bluish atmosphere which surrounds the 

 ball enlarges by several centimetres at a pressure of 2™"\ and its exterior out- 

 line is covered with small filaments, presenting a tuftlikc appearance. These 

 filaments are probably formed by the series of molecules Avhich transmit the 

 discharge. They are much more distinct with hydrogen (a good conductor) 

 than with other gases. If the ball serves as a positive electrode, it is surrounded 

 with a lively rose-colored halo of about a centimetre in diameter, presenting 

 well-marked stratifications ; then comes a dark annular space, which terminates 

 at the ring, which is itself completely invested with an envelope or sheath of 

 clear violet, with opaline tints. 



Nitrogen presents the same phenomena as hydrogen, though the stratification 

 of the electric light does not begin, except under a much feebler pressm-e. In 

 the long tube (one metre in length) the agitation of the striae, unclera pressure 

 of 2™™, is even more considerable than with hydrogen. These striae seem to 

 form an animated helix, with a movement of rotation around its axis. The 

 light is also more vivid, the tint being of a peach-blossom rather than pale rose, 

 color. The phenomenon is of a most brilliant description. Further, there is 

 the same obscure space in the vicinity of the negative electrode, the same ■ 

 glimmer of palish rose color at a weak pressure of from 1 to 2™™ in this obscure 

 space, the same appearance in this glimmering mist of well-defined and motion- 

 less rings more luminous than the space which surrounds them. 



Atmospheric air corresponds in its phenomena with nitrogen. I have ob- 

 served only that here the agitation of the stria? is less striking, and the light of 

 a rose color less deep than in nitrogen. 



Thf appearances which I have just described are, therefore, within some 

 mere shadings, precisely the same in hydrogen, nitrogen, and atmospheric air ; 

 they are equally the same, whether these gases are dry, or contain the vapor 

 of water or of alcohol in more or less quantity ; there are no difiereuces, except 

 that the pressures at which the various phenomena, and the tints of light which 

 accompany them, are observed, vary with the nature of the rarefied elastic fluid. 

 We cannot, then, attribute the effects just considered to an electro-chemical 

 decompo.^ition which cannot take place in a simple and w^ell-desiccatcd gas, nor 

 to any action appertaining to the chemical nature of the elastic fluid. They 

 are evidently the result of a mechanical action which accompanies the trans- 

 mission of electricity — an idea first advanced by M. Riess, who showed that an 

 analogous phenomenon presents itself, under a little difi'erent form, it is true, in 

 liquids and in solids. 



The phenomenon in rarefied elastic fluids Avould consist in the alternate con- 

 tractions and dilatations of the gaseous medium produced by the series of dis- 

 charges, always more or less intermittent, of which the electric stream is formed. 

 In fact, whether it be by llulunkorffs apparatus, or an ordinary electric machine, 

 or by a hydro-electric machine of Armstrong, that the stratifications are pro- 

 duced, there is never a continuous discharge, but, in reality, a series of dis- 

 charges which may succeed one another so rapidly that the intermission shall 



