THE PROPAGATION OF ELECTKICITY. 183 



The following is the result of an experiment in which the duration of the 

 passage of the current was prolonged beyond 2 minutes, through atmospheric 

 ail' at a pressure of 5°^'^^ : 



Duration of passage. Positive thermometer. Negative tlaermomcter. Difference. 



2' 18° to 31° 18° to 26° 5° 



4' — to 37° — to30i°... 6i° 



6' — to 40° — to 32° 8° 



8' — to 42° — to 33° 9° 



10' — to 43° — to 34° 9° 



In proportion as the diiratiou of the experiment increases and the absolute 

 temperature rises, the diiferences between the temperatures indicated by each 

 of the two thermometers become proportionally less ; the indications of the two 

 thermometers end by approximating, and even becoming the same after the 

 lapse of a certain time. Hydrogen and niti'ogcu give the same results. 



The numbers which express the temperatures in the preceding tables cannot 

 be gi^en as being of perfect exactness ; they vary, in effect, in their absolute 

 values with the intensity of the electric stream ; but they are sufficiently con- 

 stant and exact to demonstrate : 1st, that there is a sensible elevation of tem- 

 perature, which accompanies the propagation of the electric current in rareiied 

 gases ; 2d, that this elevation is sensibly less in the neighborhood of the 

 negative electrode than near the positive, when once the gases are sufficiiently 

 rarefied for the discharge to pass easily and the electric light to be stratified ; 

 3d, that the absolute elevations of temperature at the two electrodes, and their 

 differences, vary with the density and the nature of the gas. 



A fact which shows well all the calorific and luminous power off electricity, 

 is, that hydrogen reduced to li™'-^ of pressure can become luminous and be 

 heated in a very sensible degree* by the passage of electricity, although at that 

 pressure it has a density so inconsiderable that a cubic centimetre of the gas 

 does not weigh more than barely 5 y'jpjj of a milligramme. 



When we see matter so subtle as hydrogen reduced to one or two millimetres 

 of pressure, becoming luminous under the influence of electricity, the temptation 

 can hardly be resisted of surmising an analogy with the matter at once so subtle 

 and so luminous which constitutes the comctary bodies. This analogy becomes 

 still more striking when we examine closely the appearance presented, in the 

 tube which contains the rarefied hydrogen traversed by the electric current, by 

 those species of luminous mists which manifest themselves at the moment when 

 we introduce a little gas into the tube, and which we also see in the obscure 

 space when a certain degree of rarefaction has been attained. Undoubtedly the 

 gaseous matter is there still more rarefied than it is in the rest of the mass where 

 it is already extremely so, and it offers at the same time a remarkable resem- 

 blance with the luminous matter Avhich constitutes the comets. 



^ IV.— INFLUENCE OF MAGNETISM ON ELECTRIC CURRENTS PROPAGATED 

 IN HIGHLY Ri\JREFIED GASEOUS MEDIUMS. 



This influence, whose existence I have shown under the form of a rotation 

 communicated by the pole of a magnet to the electric currents which radiate 

 from it, is, as might be expected, and, as M. Plucker has evinced by several re- 

 markable experiments, general. The luminous filaments which display them- 

 selves in rarefied gases, traversed by the discharges of the E-uhmkorft' apparatus, 



* The heating of the gas must in fact be very considerable to be capable, in two minutes, of 

 raising by nearly 3° the temperature of a thermometer whose reservoir is a cylinder of mer- 

 cury )ii millimetres in diameter by 3 centimetres in length. Besides, the single fact that the 

 gas is luminous well evinces its high temperature ; for the light is evidently but the effect of its 

 incandescence. 



