through Rarefied Air. 315 



appeared more quiet and uniform than in the other case. After- 

 wards I examined the luminous phaenomena more minutely. 

 Before having some experience with respect to the above phaeno- 

 mena in the case of the simplest electric current, such as that 

 of a Leyden battery, it appeared to me useless to experiment 

 further upon that most complicated of all electric currents which 

 the induction apparatus furnishes. 



Experiments on the Lei/den Battery. 



I had the follov/ing- simple and convenient apparatus prepared. 

 A cylinder of very thick glass, 3| inches high and 1"46 inch 

 in interior diameter, was ground perfectly plane at one end, and 

 closed at the other by a brass mounting, which carried a cock 

 and a screw for the purpose of being screwed on to the air-pump. 

 Projecting into the glass vessel and attached to this mounting is 

 a slight brass cylinder, in which a brass rod, carrying a knob at 

 one end, can move up and down. The cylinder can be closed 

 and made air-tight by placing on the top a glass plate smeared 

 with lard. The glass plate has a small hole in its centre, and 

 upon it an ivory rod an inch long is cemented, which carries a 

 clamp for fixing the conducting wire. A piece of platinum wire 

 half a millim. thick descends from the clamp through the ivory 

 and is cemented into the glass plate, the latter and the end of the 

 wire being ground into one plane. In the plane of the lower 

 surface of the glass, therefore, there is a disc of platinum half a 

 millim. in diameter, between which and the brass knob beneath 

 it electricity can pass. After placing the knob (f ths of an inch in 

 diameter) at a distance of |ths of an inch from the platinum 

 surface, the air in the cylinder was rai'efied to about one line of 

 mercury pressure, and the apparatus, together with a sensitive 

 electric thermometer, brought into the circuit of an electric bat- 

 tery consisting of three jars. The apparatus M^as so connected 

 with the discharging wire, that during the discharge the knob 

 in the cylinder could be made either the positive or the negative 

 electrode. In order that the manner of charging the battery 

 may be seen, the part of the cylinder which was in connexion 

 with the interior of the battery is given in the following table. 

 The words " surface -f- " show, therefore, that the battery was 

 charged with positive electricity ; " knob — " that it was charged 

 with negative ; with respect to the direction of the current, it 

 will be seen that it Avas the same in both these cases. 



The following single observations show the degree to which 

 the thermometer was heated with different charges of the bat- 

 tery ; the knobs of the instrument for measuring the quantity 

 of electricity in the battery were half a line distant from one 

 another. 



Y3 



