In this state, the picture may be easily wiped off the plate. The next operation consists in gilding and fixing the picture.
Place the plate (still wet) on a levelling stand furnished with screws. Then pour upon it the following solution of sel d'or :— 20 ozs. distilled water, 1 gramme (15 grains) sel d'or.
Pour the solution into a small funnel having a tuft of cotton wool in the neck, and let it ffiter drop by drop upon the plate. Cover the plate with as much fluid as it will hold.
Light a spirit lamp with a large wick, and holding it underneath the plate, heat the fluid to the boiling point. Do not let the flame remain for two seconds on the same spot, but pass it rapidly from plac,e to place, and from corner to corner. Watch the changes of tone which the picture assumes. After a few seconds it darkens slightly, and then begins to clear up. The lights become whiter and the shadows blacker. After a time bubbles show themselves in the boiling fluid. The toning and fixing has now about reached its maxi mum. Do not let any bubbles adhere to the plate ; a spot would occur wherever this happened. To prevent this, strike the table every now and then with your left hand, t,o communicate vibration to the plate, and detach the bubbles.
When the picture has been sufficiently toned, take it by one cor ner in a pair of nippers and pour off the fluid. Pour on it distilled or ffitered rain water out of a bottle as before, and then dry it by applying a spirit lamp to the back, in the following manner :— Hold it in the nippers, inclined at an angle of about 45° to the floor ; begin with drying the upper comer, and proceed gradually downwards. As the fluid dries off, a line of wet gradually recedes downwards before the lamp ; follow this up by blowing on the plate, and remove the last drop from the lower comer by a piece of blot ting paper.
If the amateur finds a difficulty in drying the plate, he may let it dry spontaneously, in a vertical position, resting on blotting paper.
The picture is now finished. It cannot be rubbed off with the finger ; it is fast and permanent. Put it at once in the passe-par tout. But before sealing it up, blow off any dust there may be upon it, by means of an indian rubber syringe, kept for the purpose.
It sometimes happens that the plate on drying becomes covered with minute black spots. These may be removed by a very simple method. Immerse the plate in water, put it on the levelling stand, and pour on it a clilute solution of cyanide of potassium. This will
remove the spots. Wash it in water, and dry it as before. Some operators fix with cyanide instead of hypo.
The room in which the plate is sensitized should not be colder than that in which the picture is taken, or than the external air when taking a view ; for if a cold plate be taken into a warm situation the dew condenses upon it, and although a picture may be obtained in the mercury box, (and apparently a good picture,) it is neverthe less impossible to fix or tone that picture properly, and it assumes a greenish tint, and is easily rubbed off the plate.
In order fully to appreciate the beauty of a daguerreotype, it should be viewed in sunshine with your back to the light. The ultimate tint depends in great measure on the observance of the pro portions of iodine and bromine in the sensitive coating.
A dag,uerreotype is permanent ; the picture c,annot be rubbed off the plate ; the plate caimot be broken by aceident ; the picture will bear microscopic investigation. Other processes have their merits, but this is the triumph of the photographic art, and a boon to science.
The theory of this process is so exceedingly obscure and uncer tain that at present any attempt at explanation of it must involve much that is hypothetical.
The sensitive film is supposed to be at first in an amorphous state, big to be crystallized and roughened by the action of light. The mercurial vapour adheres to this roughened sudace and forms the lights of the picture by amalgamating with the silver. The iodine and bromine are removed by the hyposulphite of soda. The boiling solution of sel d'or appears to act by elective affinity, in the follow ing manner :—Sel d'or is a double hyposulphite of gold and soda. An atom of silver of the plate changes places with an atom of gold of the solution. The sel d'or therefore beoomes converted into a double hyposulphite of silver and soda, and gold is precipitated. The .gold, silver, and mercury form an amalgam which adheres tightly to the plate, assisted perhaps by the expansion and subse quent contraction of the metal on cooling. This forms the lights of the picture, purple under one aspect, and golden under another. The shadows, where no mercury exists, are blackened and enriched by the precipitation of gold.
Such appears to be the theory of this very beautiful process.