RECENT PROGRESS IN PHYSICS. 



393 



remained suspended 



paratus tlie support was perforated and furnished witli a stop-cock ; it 

 could be screwed to an air pump, a vacuum made, and any other gas 

 introduced. 



Earifying and heating tho air produced no change in its specific in- 

 ductive capacity. 



Faraday made further experiments for the purpose of establishing 

 his views on this subject. 



Let A, fig. 37, be an insulated metallic plate placed between two 

 other metallic plates, B and C, insulated in like i -. ■'". 



manner, B and C being each !^ inch distant from 

 A. With C a wire was connected which termi- 

 ted in the gold leaf c, and in like manner a wire 

 fastened to B terminated in the leaf h. The 

 two gold leaves hung in a glass jar two inches 

 apart. B and C were then connected with the 

 ground and a weak positive charge given to the 

 plate A, by means of which B and C were charged 

 with — E. The connection of B and C with the 

 earth was then cut off, so that these two plates 

 were again insulated — the gold leaves h and c 

 parallel to each other as before. 



A shellac plate, f inch thick and 4 inches square, suspended by 

 a clean thread of white silk, after being carefully deprived of all 

 charge, was brought between the plates A and B. The electric rela- 

 tions of the three plates were at once altered, and attraction was pro- 

 duced between the gold leaves. On the removal of the shellac this 

 attraction again disappeared. The shellac having been then examined 

 by a sensitive CoulomFs electrometer, indicated no charge. 



In this FaradaTj found a further confirmation of his views, and he 

 explained the result as follows : As soon as the shellac plate is intro- 

 duced between A and B a strong charge of negative electricity takes 

 place on B — it repels the positive, which is thus difi'used towards h ; 

 but, since A acts more powerfully on B than before, negative electri- 

 city on C must be set free ; thus c will contain free — E, while free 

 -|-E is on h ; hence the attraction of the leaves. 



How Faraday has proved that this electricity is set free on h and c, 

 as it must be according to his theory, does not appear in his memoir. 



Faraday's experiments are perfectly correct, but it appears to me 

 that he has erroneously interpreted these experiments and drawn a 

 conclusion from them in which he is not justified. The grounds for 

 this assertion are as follows : 



If an insulated electrified body A is placed opposite a second con- 

 ductor B, which is in communication with the ground, a definite quan- 

 tity of electricity will be rendered latent on B. A part of the E on 

 A is then disguised by the opposite kind on B^ and a part is free. If 

 shellac is now placed between A and B, more electricity is disguised 

 on A, and there is less free than before ; this is the fact which is ex- 

 hibited by the experiments of Faraday with the inductive apparatus. 

 He further asserts, however, that a stronger induction takes place 

 through the shellac, but this he has not proved by experiment. To be 

 justified in this assertion he should have shown, that with equal charges 



