220 DISCOVERIES OF MR. HUNT. SECT. XXIV. 



Professor Moser proved that bodies exert a very decided influ- 

 ence upon each other, by placing coins, cut stones, pieces of horn, 

 and other substances, for a short time on a warm metallic plate : 

 when the substance was removed, no impression appeared on the 

 plate till it was breathed upon or exposed to the vapour of mer- 

 cury, and then these vapours adhered only to the parts where the 

 substance had been placed, making distinct images, which in 

 some cases were permanent after the vapour was removed. 

 Similar impressions were obtained on glass and other substances 

 even when the bodies were not in contact, and the results were 

 the same whether the experiments were performed in light or in 

 darkness. 



. Mr. Grove found, when plates of zinc and copper were closely 

 approximated, but not in contact, and suddenly separated, that 

 one was positively and the other negatively electric ; whence he 

 inferred that the intervening medium was either polarised, or 

 that a radiation analogous, if not identical, with that which pro- 

 duces Moser's images takes place from plate to plate. 

 . Mr. Hunt has shown that many of these phenomena depend 

 on difference of temperature, and that, in order to obtain good 

 impressions, dissimilar metals must be used. For example, gold, 

 silver, bronze, and copper coins were placed on a plate of copper 

 too hot to be touched, and allowed to remain till the plate 

 cooled: all the coins had made an impression, the distinctness 

 and intensity of which were in the order of the metals named. 

 When the plate was exposed to the vapour of mercury the result 

 was the same, but, when the vapour was wiped off, the gold and 

 silver coins only had left permanent images on the copper. 

 These impressions are often minutely perfect, whether the coins 

 are in actual contact with the plate or one-eighth of an inch 

 above it. The mass of the metal has a material influence on the 

 result ; a large copper coin makes a better impression on a copper 

 plate than a small silver coin. When coins of different metals 

 are placed on the same plate they interfere with each other. 



When, instead of being heated, the copper plate was cooled by 

 a freezing mixture, and bad conductors of heat laid upcn it, as 

 wood, paper, glass, &c., the result was similar. 



Mr. Hunt, observing that a black substance leaves a stronger 

 impression on a metallic surface than a white, applied the pro- 

 perty to the art of copying prints, woodcuts, writing, and 



