242 Prof. De la Rive on the Propagation of Electricity 



Mr. Gassiot has succeeded in giving a direct and rigorous 

 proof of this principle, by producing an almost absolute vacuum 

 in tubes previously filled with rarefied carbonic acid, by intro- 

 ducing into them a fragment of potash which was allowed to cool 

 after having been heated. 



The talented English physicist has since obtained a confirma- 

 tion of his first results by using a water-battery of high tension 

 instead of the induced current of RuhmkorfFs apparatus*. It 

 may therefore be regarded as completely proved that an abso- 

 lute vacuum does not permit the passage of electricity, and that 

 this passage can only take place through the agency of a ponde- 

 rable medium. This medium may be of extreme tenuity, it is 

 true ; but its presence is not therefore the less essential for the 

 transmission of electricity. 



Thus elastic fluids, which had long been considered such 

 imperfect conductors of electricity that they were classed among 

 insulators along with glass or resin, are capable, when reduced 

 to a sufficient degree of rarefaction, of transmitting electric dis- 

 charges of sufficient force to affect a magnetized needle, and to 

 be acted upon in their turn by magnets in the same way as vol- 

 taic currents. 



The transmission of electricity through elastic fluids presents 

 certain special characteristics which render it one of the most 

 important physical phenomena. 



In the first place, the fact that gases do not conduct electri- 

 city until they are reduced to a certain degree of rarefaction, ap- 

 pears at first sight incompatible with the principle that an abso- 

 lute vacuum is a non-conductor ; for if electricity is propagated 

 by matter, it seems as if the propagation ought to take place so 

 much the more easily in proportion as the matter is in larger 

 quantity. It is not so, however; and experiment shows that 

 between a vacuous space and a space filled with gas at the 

 atmospheric pressure, there is a degree of rarefaction of the gas 

 (for example 2*5 millims.. for hydrogen) at which it possesses a 

 maximum of conductivity or a minimum of resistance; and this 

 appears to prove that the mutual disposition of the particles, in- 

 dependently of their particular nature, exerts great influence on 

 the conducting-power of the gaseous medium. 



Another not less important characteristic of the propagation 

 of electricity through elastic fluids is the production of stria?, or 

 the stratification of the electric light, which occurs when the 

 gaseous medium is brought to such a degree of rarefaction that 

 the discharge or current passes easily. This stratification was 

 attributed at first to the employment of induced currents, and to 

 the opposition of the currents succeeding each other in contrary 

 •* Proc. Roy. Soe. vol. xii. p. 329, December 18, 1862. 



