The Daguerreotype process, discovered by M. Daguerre in 1839, has been entirely superseded by the easier, healthier, and less expensive collodion processes described further on. It consists in submitting a plate of silver or silvered copper to the vapour of iodine and bromine in the dark. A sensitive film of iodide and bromide of silver is thus formed on the plate, which is immediately exposed to the image of an object in the camera obscura. The latent image impressed on the plate is brought out or developed by exposing the plate to the action of the vapour of mercury.
It is to be noted that in all English photographic formulæ the solid and fluid measures
of apothecaries weight are used; but in buying or selling chemical articles, the avoirdupois weight is employed.
Nitrate of Silver Bath (for Positives). Recrystallised nitrate of silver, 5 dr., dissolved in 10 oz. of distilled water. Filter the solution until it is quite clear, then add 3 drops of nitric acid and 10 drops of collodion. Shake well together and filter. Blue litmus paper should slightly redden in this bath; should it turn very red, add a little ammonia, or oxide of silver; should it not redden at all, add a little acid carefully drop by drop. It is preferable to have a slight excess of acid.
Developing Solution (for Positives). Protosulphate of iron, 2 dr., dissolved in 8 oz. of distilled water; add 21⁄2 dr. of glacial acetic acid, 21⁄2 dr. of alcohol, and 5 minims of nitric acid, filter, and pour into a well-stoppered bottle; do not expose to the air.
Fixing Solution (for Positives). 50 gr. of cyanide of potassium dissolved in 5 oz. of distilled water; that is to say, for every fluid ounce of solution required, mix 10 gr. of cyanide of potassium in 1 oz. of distilled water. Filter and keep in a well-stoppered bottle, and label “Poison.”
Positive Paper. Plain paper requires preparing or salting before it is ready for use, or it may be purchased already salted.
Procure some sheets of plain Saxe paper, and immerse them for five minutes (removing air-bubbles) in the following solution:—
| Chloride of ammonium | 100 | gr. |
| Chloride of barium | 100 | gr. |
| Citrate of soda | 20 | gr. |
| Water | 20 | oz. |
Hang the sheets up to dry. For portraits and most other uses the paper is albuminised on one side. When photographs are printed to be afterwards coloured, unalbuminised paper is used.