74 CORRELATION OF PHYSICAL FORCES. 



I have succeeded in farther extending this experiment, and 

 in permanently fixing the images thus produced by electricity. 

 Between two carefully cleaned glass plates is placed a word or 

 device cut out of paper or tinfoil ; sheets of tinfoil a little 

 smaller than the glass plates are placed on the outside of each 

 plate, and these coatings are brought into contact with the 

 terminals -of a Rhumkorf coil. After electrisation for a few 

 seconds, the glasses are separated, and their interior surfaces 

 exposed to the vapour of hydrofluoric acid, which acts che- 

 mically on glass ; the portions of the glass not protected by 

 the paper device are corroded, while those so protected are 

 untouched or less affected by the acid, so that a permanent 

 etching is thus produced, which nothing but disintegration of 

 the glass will efface. 



Some farther experiments of mine on this subject bring 

 out in a still more striking manner these curious molecular 

 changes. One of the plates of glass having been electrified 

 in the manner just mentioned, is coated, on the side impressed 

 with the invisible electrical image, with a film of iodised col- 

 lodion in the manner usually adopted for photographic pur- 

 poses ; it is then in a dark room immersed in a solution 

 of nitrate of silver ; then exposed to diffuse light for a few 

 seconds. On pouring over the collodion the usual solution of 

 pyrogallic acid, the invisible electrical image is brought out 

 as a dark device on a light ground, and can be permanently 

 fixed by hyposulphite of soda. The point worthy of observa- 

 tion in this experiment is, that this permanent image exists 

 in the collodion film, which can be stripped off the glass, dried, 

 and placed on any other surface, so that the molecular change 

 consequent on electrisation has communicated, by contact or 

 close proximity, a change to the film of collodion correspond- 

 ing in form with that on the glass, but undoubtedly of a che- 

 mical nature. Electricity has, moreover, in this experiment 

 so modified the surface of glass, that it can, in its turn, mo- 

 dify the structure of another substance so as to alter the 

 relation of the latter to light. It would require a curious 

 complication of hypothetic fluids to explain this ; but if elec- 

 tricity and light be supposed to be affections of ordinary 

 ponderable matter, the difficulty is only one of detail. 



