and its Effects. 131 



was already considerably greater than that at which sparks would 

 pass between the metal plates when they were alone in air with- 

 out the intervention of glass. 



The influence of the glass in transmitting the action of induced 

 electricity was consequently plainly proved. The following ex- 

 periments will furnish additional evidence of this. The upper 

 round copper plate was kept insulated, and held at a distance of 

 7 milhms. from the lower plate on the table. Now and then a 

 single, broad, short spark darted across where the distance be- 

 tween them was least, for without great care the upper plate 

 could not be placed exactly parallel to the lower one. 



A glass plate, 4 millims. thick, was now placed upon the lower 

 metal. Immediately a perfect shower of small sparks fell from 

 the upper plate upon the glass. This change in the nature of 

 the sparks did not, as one might imagine, arise from the fact 

 that the distance was now diminished to 3 millims. ; for when 

 the lower system was inverted, i. e. when the glass plate was 

 laid upon the table and covered with the copper plate, the sparks 

 between the latter and the upper plate were again of the same 

 kind as before, although, of course, smaller and more frequent 

 than at the original distance of 7 millims. 



The above-mentioned change, therefore, is an effect of the 

 glass ; and this is also evident when the latter does not at all 

 touch the metals, but is held in free air between them. Small 

 sparks are seen to spring from both metal plates towards the 

 glass, at distances where, in absence of the latter, no sparks 

 would appear. 



The same effects are produced when other insulating substances 

 are used instead of glass, e. g. plates of marble, gutta-percha, 

 broad columns of liquids, such as distilled water, alcohol, or oil 

 of turpentine, enclosed between glass plates. The electrometer 

 proves that in all these cases the insulating substance receives 

 no, or only a very weak and indefinite, permanent charge. 



A pointed copper wire was now substituted for the small, 

 round, copper plate, and, like it, connected with one pole of the 

 induction coil. When held at a distance above the greater plate, 

 the point of this wire appeared luminous in the dark ; the weaker, 

 of course, the greater the distance. With the power employed 

 by me in all these experiments, viz. that of two of Grove's ele- 

 ments in combination with the shorter of my induction wires, 

 the luminosity at the distance of 2 inches was so weak that it 

 was scarcely perceptible. If now, when the wire is held at this 

 limiting distance, the above-mentioned system of glass plates, 

 half an inch in thickness, be laid upon the lower plate, the lumi- 

 nosity of the point will immediately be again distinctly visible ; 

 it increases the nearer the point approaches the glass, until, at a 



