A Manual Of Photography

THEHMOGRAPHY. 167 each piece had made its impression, but those made by the gold and the large medal were more distinct ; not only was the disc marked, but the lettering on each was copied. 4. A bronze medal was supported upon slips of wood, placed on the copper, one eighth of an inch above the plate. After mercurialisation, the space the medal covered was well marked, and, for a considerable distance around, the mercury was unequally deposited, giving a shaded border to the image ; the spaces touched by the mercury [?] were thickly covered with the vapour. 5. The above coins and medals were all placed on the plate, and it was made too hot to be handled, and allowed to cool without their being removed ; impressions were made on the plate in the following order of intensity,—gold, silver, bronze, copper. The mass of the metal was found to influence ma- terially the result ; a large piece of copper making a better image than a small piece of silver. Wlien this plate was exposed to vapour, the results were as before. On rubbing off the vapour, it was found that the gold and silver had made per- manent impressions on the copper. 6. The above being repeated with a still greater heat, the image of the copper coin was, as well as the otliers, most faith- fully given, but the gold and silver only made permanent im- pressions. - . 7. A silvered copper plate was now tried with a moderate warmth. Mercurial vapours brought out good images of the gold and copper ; the silver marked, but not well defined. 8. Having repeated the above experiments many times with the same results, I was desirous of ascertaining if electricity had any similar effect : powerful discharges were passed through and over the plate and discs, and it was subjected to a long-continued current without any effect. The silver had been cleaned oft from the plate ; it was now warmed with the coins and medals upon it, and submitted to discharges from a very large Leyden jar: on exposing it to mercurial vapour, the impressions were very prettily brought out, and, strange to say, spectral images of those which had been received on the plate when it was silvered. Thus proving, that the influence, whatever it may be, was exerted to some depth in the metal. 9. I placed upon a plate of copper, blue, red, and orange- coloured glasses, pieces of crown and flint glass, mica, and a square of tracing paper. These were allowed to remain in con- tact half an hour. The space occupied by the red glass was well marked ; that covered by the orange was less distinct, but the blue glass left no impression; the shapes of the flint and crown