The picture is made, and has now to be preserved. It is necessary first to re move the superfluous iodine and bro mine from the sides of the picture, where the light has not produced any chemical action ; this is accomplished by washing with hyposulphate of soda, a salt which is capable of dissolving the iodide of sil ver formed on the plate. The washing should be performed immediately after the exposure to the mercury. The plate is held by a pliers in the hand, and the hyposulphate solution is poured over the plate, and washed around it. The plate is then rinsed with water, and dried off by the heat of a spirit lamp applied underneath. The hyposulphate solution should be weak.
The picture is now formed, and the su perfluous coating removed : it has yet to be fixed. This is accomplished by gild ing, or applying a weak solution of chlo ride of gold, washed over the plate. This protects it from any further action of light, rendering the image permament. The solution of the chloride of gold is poured on the plate, heated beneath by the lamp, and allowed to remain on as long as any small bubbles continue to appear.
The pictures arc usually colored by means of mineral powders, laid on dry with a brush. Yellow ochre burned is the usual flesh color, mixed with carmine or chrome yellow. Oxide of bismuth forms the white ; Prussian blue, the bine ; and the green is formed by the mixture of blue and yellow.
When the silver plate is coated, an io dide of silver is produced. When it is further exposed to bromine, a portion of bromide of silver is also formed, so that the plate is then covered with two pre parations of silver, the bromide and the iodide ; the latter is the salt which it is desirable to have formed upon the plate, and all applications have for their object the ready formation and decomposition of it. When exposed to the light the io dide of silver is decomposed in some places, while in other places the decom position is not effected. When the plate already acted on by light is exposed to the mercury, the latter coats these places where the light has acted on so that the light parts of the picture are an amalgam of mercury while the dark places are of silver ; the intermediate tints are a mix ture of the two. The daguerreotype as at present produced shows only the ef fect of lights and shade. It has not yet satisfactorily produced color. Occasion ally indeed, in the hands of an artist, a single color is produced, and Becquerel, by the aid of a galvanic travers ing the plate while in the camera, has produced an occasional color (not the na tural one). Mr. Hill, of Greene Co.
N. Y., has been asserted to have pro duced the colors of nature, andplates so colored have been seen by a few but whether from uncertainty in the use of the materials, or want of artistic finish in the plates so produced, the publication has not yet appeared. It may not be amiss to point out the road to success— it lies in the taking images more rapidly more sensitive plates, and more powerful accelerators must be used. It is very probable that a polished surface is not that which is capable of receiving the co lors • a surface chemically pure, yet not capable of reflection, will be likely to an swer better ; hence the newly electro typed surface should be most effective, and when this is excited by more sensi tive coatings than the present, it is likely that the colors of nature will be reflected as they impinge. There is no doubt that the coating with mercury covers these up, hence this must ultimately be dis pensed with, as also the use of cloride of gold. Chloro-cromic acid is a powerful accelerator, and may be useful in the first stages ; but the whole difficulty has not as yet been solved.
Daguerreotype plates may be etched and thus the art usefully applied to objects of natural history. To do this it is neces sary to etch away the dark parts and leave the white untouched. Theplate is immersed in a fluid consisting of dilute nitric acid, nitrous acid, chloride of sodi um, and nitrate of potass. These two salts are decomposed when the fluid is heated and chlorine and nitrous acids are evolved, these attack and remove the silver or dark parts, but have no effect on the mercury, so that the lights of the pic ture form the etchingground, and pro tect these portions of the plate. Am monia is used to wash the plate and re move the cloride of silver formed, and allow the etching being carried on far ther. The plate is now inked and allowed to dry, the surface is then polished, and gilded by the electrotype, those parts only taking the gold which had been polished previously. The plate may now be still further etched with nitric acid; the ink having been washed out by pot ash and the etching carried on until it has gone sufficiently deep. M. Claudet has obtained some beautiful engravings of the lower animals by this process.