20 Prof. E. Edlund on the Path of Electrical Induction* and 



Hence the deflection is always greater in dry air than in that 

 which is saturated with moisture. Even when dry air was com- 

 pared with air in the working-room, which was far from being 

 saturated with moisture, the deflection was greater in dry air. In 

 an experiment on this point a deflection of 8 divisions was ob- 

 tained for the dry air, and for the undried air a deflection of 6*1 

 divisions. 



It does not seem very easy clearly to make out the causes why 

 the greater deflection was obtained with the spark in dry air. 

 The following circumstance would have to be taken into account. 

 It is necessary for the formation of the sparks in dry air that 

 the electric density upon the pole-surfaces be greater than 

 when the air is moist. Before the formation of sparks, the 

 electrical density increases upon the polar surfaces until it is 

 great enough to traverse the layer of air between them. Hence 

 in moist air the spark appears sooner and with a smaller electrical 

 density. The disintegration of the polar surfaces thereby be- 

 comes smaller, and consequently there is a diminution in the 

 electromotive force of the disjunction. As the conducting- 

 power of the spark doubtless depends on the quantity of detached 

 metallic particles, this will also be smaller. Now, if a diminution 

 of the electromotive force and of the conducting-power take place 

 when the air is moist, a decrease in the magnitude of the de- 

 flection must necessarily follow. The deportment with rarefied 

 gases seems to speak in favour of the same mode of explanation. 



Experiment 16. — The glass cylinder was filled with carbonic 

 acid, exhausted, and these operations repeated several times until 

 it was certain that no air was left. The carbonic acid, which 

 was prepared from marble and hydrochloric acid, and was ascer- 

 tained by testing to contain scarcely perceptible traces of foreign 

 gases, was dried before entering the apparatus previously men- 

 tioned. The polar cones in the glass cylinder had to be pushed 

 very near together, because otherwise the spark would not pass 

 when the cylinder was filled with carbonic acid. This is the 

 reason why the deflections were relatively small. 



With carbonic acid the following deflections were observed: — 



Deflections. 

 163 

 13-3 

 15-3 

 16-8 



Mean . . 15*4 



Experiment 17. — -The glass cylinder was filled with air (un- 

 dried) : — 



