150 Intelligence and Miscellaneous Articles. 



the jar of which the air had retained the pressure of 13 millims. 

 The small differences of velocity observed in the jar in which the 

 pressure remained the same were occasioned by this — that the aug- 

 mentation of pressure in the other, of course, diminished somewhat 

 the intensity of the current transmitted successively through the 

 two jars. 



This experiment (repeated several times, and ahvays with the 

 same result, though under different forms) shows that the velocity 

 of rotation of the jet varies, ccBteris paribus, with the density of the 

 gaseous medium, but that it diminishes in a less ratio than that in 

 which the density increases. 



Carbonic acid gave the same result as atmospheric air. The gas 

 being at 30 millims. pressure in one of the jars, and at 15 millims. 

 in the other, the naraber of turns of the jet in 30 seconds was 30 in 

 the first, and 50 in the second. 



One of the jars being filled with air, and the other with carbonic 

 acid, and the two gases being at the same pressure of 20 millims., 

 the number of turns in 30 seconds was 40 in the air and only 30 in 

 the carbonic acid — which shows that the density of the gas, inde- 

 pendently of the pressure, exerts a great influence on the velocity of 

 rotation. 



We hkewise experimented on some other gases ; but, with the 

 exception of carbonic acid, the compound gases, being rapidly de- 

 composed by the electric jet, cannot give exact results. Hydrogen 

 is not in the same category ; but in it the velocity of rotation is so 

 great that it is very difficult to appreciate it directly. We shall 

 afterwards return to the results given by the employment of this gas. 



These first experiments having shown the resistance opposed to 

 the electric jet by the m.edium in which it moves, we were led to 

 try w^hat resistance would be exerted upon it by a moveable solid 

 obstacle. 



With this view we suspended in a wide and high bell-glass, by 

 means of a cocoon-thread, a small square of gumm.ed paper disposed 

 so as to present its vertical face to the action of the horizontal elec- 

 tric jet. Every time that the jet, in its rotation under the action of 

 the magnetism, encountered the square of paper, this received an 

 impulse, renewed at each passage of the jet, so that at last it oscil- 

 lated like a pendulum. The air in w hich the experiment took place 

 was at from 15 to 20 millims. pressure. 



For a better study of this mechanical action of the jet, we replaced 

 the little pendulum by an ivory swivel wdth an agate socket, move- 

 able on a pivot placed in the centre, so that it could revolve in a 

 horizontal plane, parallel with the jet, but a little below it. Each 

 of the two extremities of the ivory needle carried a vertical disk of 

 light glass 5 centims. in diameter, which the electric jet in its rota- 

 tion touched in passing. Thence the swivel received an impulsion 

 which ended in impressing on it a movement of rotation, the velo- 

 city of which went on for a long time increasing, and only became 

 constant when the resistance of the surrounding rarefied air and the 

 friction on the pivot balanced the accelerating force. 



