86 CORRELATION OF PHYSICAL FORCES. 



consequent upon electrisation : A coin is placed on a pack of 

 thin plates of glass, and then electrified. On removing the 

 coin and breathing on the glass plate, an impression ^)f the 

 coin is perceptible ; this shows a certain molecular change on 

 the surface of the glass opposed to the plate, or of the vapours 

 condensed on such surface. This effect might, and has been 

 interpreted as arising from a film of greasy deposit, supposed 

 to exist on the plate ; the impressions, however, have been 

 proved to penetrate to certain depths below the surface, and 

 not to be removed by polishing. 



The following experiment, however, goes farther : On 

 separating carefully the glass plates, images of the coin can 

 be developed on each of the surfaces, showing that the mole- 

 cular change has been transmitted through the substance of 

 the glass ; and we may thence reasonably suppose that a 

 piece of glass, or other dielectric body, if it could be split up 

 while under the influence of electric induction, would exhibit 

 some molecular change at each side of each lamina, however 

 minutely subdivided. 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 Khumkorf s coil. 

 After electrisation for a few seconds, the glasses are sepa- 

 rated, and their interior surfaces exposed to the vapour of 

 hydrofluoric acid, which acts chemically on glass ; the por- 

 tions of the glass not protected by the paper device are cor- 

 roded, 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 



