第35章

If either of the latter be used, there will be formed sulphate of potash and a carbonate of the metal used, and there will also be a considerable effervescence of carbonic acid, which will, if care is not taken, cause the mixture to run over the vessel.

After the copper or iron salt is neutralized, which is known by its ceasing to effervesce, the carbonate of the metal will settle slowly, and will at first nearly fill the vessel.

The supernatant fluid, which is sulphate of potash in solution, may now be carefully poured off, and its place filled with water;this operation should be repeated several times until the water which passes off is tasteless. The carbonate of the metal rapidly changes to an oxide by contact with the air, and it will generally be found, when it is sufficiently washed, that it is at least half oxide.

On adding hydrochloric acid cautiously to the mixture, a chloric of the metal will be formed, and carbonic acid will be evolved from the remaining carbonate. The chloride formed is soluble;but as there are two chlorides of these metals, and we wish to produce the one which contains the most chlorine, it is best to add the acid cautiously until the solution is decidedly acid.

After filtering the solution, it is fit for use; and it should be preserved in well-stoppered bottles. The water used should be rain or distilled water.

"About one part of the mixed chlorides should be used to three or four of water.

"The battery may be either Smee's, Daniell's, or Grove's;if of either of the former, it should be of two series;if of the latter, one cup is sufficient.

"The plate on being immersed in the liquid, almost instantly takes a violet color. It should be allowed to remain from two to five minutes, according to the strength of the battery, and until it becomes nearly black.

It should now be carefully washed, and afterwards heated over a spirit lamp until it takes a cherry-red color, and it is then ready for exposure in the camera. Before speaking of exposing the plate, it may be well to speak of some difficulties which the inexperienced operator may find in preparing it. If the battery is not in good order, and a sufficient current is not passed through the solution, the plate will become coated--and apparently almost as well as when the battery is working well--but on exposure it will give a negative picture, and but little colored;while if the battery is in good order, the impression is invariably positive.

"Sometimes on heating the plate after washing, the surface is covered with spots or assumes a variegated appearance.

This indicates that the solution is impure, or that the plate have not been thoroughly washed and are still contaminated with the soluble chlorides which are contained in the solution.

"From the fact that the plate if prepared with positive electricity gives a positive picture, while it prepared otherwise it gives a negative, it is evident that electricity plays an important part in this process.

The same is true to some extent with the compounds formed with iodine, bromine. and fluorine.

"On heating the plate, the brown coating of chloride melts into a translucent enamel, and the heat should be withdrawn when a cherry-red color is produced.

It the heat is continued longer, the plate assumes a lighter color, and becomes less sensitive; and the enamel will finally scale off.

To produce a picture by the ordinary process of M. Neipce, unaccelerated, it should be exposed for from three to five hours to sunlight in the camera, though pictures may be procured by contact, in from fifteen to thirty minutes."MULTIPLYING DAGUERREOTYPES ON ONE PLATE.

I have produced some interesting specimens of the Daguerreotypic art, by exposing in the camera only a portion of the sensitive plate to the action of light. When on the exposed portion an image is formed, then taking the tablet into the dark room, change ends and expose the sensitive portion, and produce another image, developing as usual.

This plan is adapted for taking likenesses for lockets.

Two images can be presented as sitting side by side, by covering half the plate with black paper, and exposing as before.

In this manner we have been enabled to surprise persons by exhibiting their portrait on the same plate with a stranger's. Daguerreotypists must be cautious in practicing this, as it might not be agreeable to the parties whose likenesses are together, by the above process.

It is impossible to produce an impression without a line being seen where the edge of the paper prevented the operation of the light.

I have recently seen a fine specimen produced by another plan, which far exceeds the above, there being no line, or any peculiarity denoting two exposures. The specimen referred to, was a gentleman represented on one plate by two full length portraits.

This was produced by using a black velvet for the background.

The plate was exposed sufficient time to produce one impression, and then the gentleman assumed another position, and is repeated as looking at himself. From the fact that the time required to develop black velvet being so much longer than that for producing a portrait, we are enabled to produce the above interesting results.

DEPOSIT IN GILDING.

Regarding specks from bad water, I would remark that gilding should be made only with distilled water. Thus made, it produces very little deposit, even by long keeping.

It therefore preserves its original strength, and works with great uniformity.

Every grain of deposit contains at least 7-10 its weight of gold, easily discoverable by the blowpipe.

Such gilding is continually deteriorating, which with good chloride and distilled water may be prevented.

Distilled water should also be used for the hyposulphite.

and for cleaning plates. Any good, clear water may be afterwards used for washing off, with equally good results.

I am very rarely troubled with specs, and deem this as the main reason.