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DAGUERREOTYPE.

the heat of a spirit flame, until the silvered surface becomes of a welldefined golden-yellow colour; then, when the plate is cold, take a pieceof cotton, dipped in very dilute nitric acid, and rub lightly over ituntil the white hue is restored, and dry it with very soft clean cloths.A weak solution of the hydriodate of potash, in which a small portionof iodine is dissolved, is now passed over the plate with a wide camel’shair brush. The silver is thus converted, over its surface, into an io-duret of silver; and in this state it is exposed to light, which blackensit. When dry, it is to he again polished, either with dilute acid, or asolution of carbonate of soda, and afterwards with dry cotton, and thesmallest possible portion of prepared chalk; by this means a surface ofthe highest polish is produced. The rationale of this process is, in thefirst place, the heat applied drives off any adhering acid, and effectsmore perfect union between tho copper and silver, so as to enable it tobear the subsequent processes. The first yellow surface appears to bean oxide of silver, with, possibly, a minute quantity of copper in com-bination, which being removed, leave a surface chemically pure. Cop-per plates may also be very beautifully silvered by galvanic agency, bywhich wo are enabled to increase the thickness of tho silver to anyextent, and the necessity for the heating process is removed, the silverbeing absolutely pure. The best and simplest mode with which I amacquainted, is to divide an earthenware vessel with a diaphragm ; oneside should be filled with a very dilute solution of sulphuric acid, andthe other with either a solution of ferroprussiate of potash, or muri-ate of soda, saturated with chloride of silver. The copper plate, var-nished on one side, is united, by means of a copper wire, with a plate ofzinc. The zinc plate being immersed in the acid, and the copper inthe salt, a weak electric current is generated, which precipitates thesilver in a very uniform manner over the entire surface.

At a very early stage of my inquiries I found that the influence ofall the rays, excepting the yellow, was to loosen the adhesion of theiodidated surface, and tho under layer of unaffected silver. When thischanged film was remove!! by rubbing, the silver beneath always ex-hibited the most perfect lustre, and I have hence invariably adoptedthis mode of polishing my Dauguerreotype plates. The required sur-face is thus produced with one-third the labour, and a very great savingof time; besides which, the silver is in a much more susceptible statofor receiving the vapour of the iodine. The plate being thus prepared,we proceed in the manner before directed.

It is somewhat singular, that on the first notice of Daguerre ’spictures, long before the publication of his process, when I learnt thatthey were on “ hard polished tablets,” I entertained the idea thatplates of copper thus silvered were oxidized, and then acted on byiodine. I applied the iodine, both in solution and vapour; but, ofcourse, as the mercury was not used, I failed to effect any perfect pic-tures. It is, however, worthy of remark, that on one occasion, havingplaced a piece of silvered copper in a trough containing a weak solution