212 MM. A. de la Rive and~$. Sarasin on the Action of 



Finally we examined in the same connexion a third case, in 

 which the electric jet is caused to rotate continuously under the 

 action of an electromagnet arranged for that purpose within the 

 rarefied gaseous medium. 



As our aim was not to search out for each gas the numerical 

 coefficients relative to those different kinds of action, but only to 

 determine the general laws of the phenomena with which we are 

 engaged, we have limited our operations to three gases very dif- 

 ferent in their physical and chemical properties, viz. atmospheric 

 air, hydrogen, and carbonic acid. 



I. Influence of Magnetism on the Density of the rarefied Gas 

 traversed by the Electric Discharge. . 



When the electric jet transmitted through a rarefied gas in a 

 tube is submitted to the action of magnetism, the jet, tending, 

 as Pliicker has shown, to describe a magnetic curve, bears towards 

 the sides of the tube (which just prevent it from describing ex- 

 actly the curve), and that to one side or the other, according to 

 the relative direction of the magnetism and of the electric cur- 

 rent. At the same time the jet, which spreads itself out more 

 or less in the tube according to the density of the gas, seems to 

 contract to a tolerably thin thread. One is therefore disposed 

 to believe that the gas itself is condensed under the action of the 

 electromagnet, and that perhaps this is the cause to which is due 

 the greater resistance it then offers to electric conduction ; conse- 

 quently we thought we ought to commence our work by endea- 

 vouring to ascertain if this condensation really takes place. 



From the first trials we made, by putting the tube traversed 

 by the electric discharge under the action of magnetism into 

 communication with a very sensitive manometer, we recognized 

 that the apparent condensation of the electric jet is accompanied 

 by an augmentation of the elastic force of the gas ; and this fact 

 is observed with the greatest facility. When the induction- 

 current is passed through a tube filled with any gas brought to 

 a certain degree of rarefaction, and communicating with the ma- 

 nometer, the latter indicates a very notable increase of pressure 

 at the moment the discharge begins to pass. This increase of 

 pressure evidently results from the heating of the gas. When 

 the tube, continuously traversed by the electric discharge, is 

 afterwards submitted to the action of magnetism, the pressure in 

 the tube is seen to diminish, but never returns quite to what it 

 was before the passage of the current. Under the action of the 

 magnetization, then, the elastic force of the gas traversed by the 

 electric discharge has been sensibly diminished, though without 

 returning to what it was before the passage of the current. This 

 diminution must be due, at least in part, to the weakening of 

 the current, which from that time must produce less heating. 



