The kind of paper on which the silver is spread is an object of much importance. A paper known to stationers as satin post, double-glazed, bearing the mark of J. Whatman, Turkey Mill, is decidedly superior to every other kind I have tried. The dark specks which abound in some sorts of paper must be avoided, and the spots made by flies very carefully guarded against. These are of small consequence during the darkening process, but when the bleaching wash is applied, they form centres of chemical action, and the bleaching process goes on around them, independently of light, deforming the drawing with small rings, which are continually extending their diameters.

The Solution Of Silver

Take of crystallized nitrate of silver 120 grains, distilled water 12 fluid drachms; when the salt is dissolved, add of alcohol 4 fluid drachms, which renders the solution opaque ; after a few hours, a minute quantity of a dark powder, which appears to be an oxide of silver, is deposited, and must be separated by the filter. The addition of the alcohol to the solution was adopted from an observation I made of its in-fluence in retarding the chemical action, which goes on in the shade, of the hydriodates on the salt of silver. Its use is, therefore, to make the action depend more on luminous influence, than would be the case without it.

Nitric Ether

Sweet spirits of nitre not only checks the bleaching process in the shade, but acts with the iodine salts to exalt the oxidation of the silver, or increase the blackness of it. In copying lace or any fine linear object, it is a very valuable agent; but it is useless for any other purposes, as all the faintly-lighted parts are of the same tint.

Hydrochloric Ether

Hydrochloric Ether, used as the solvent of the silver, and applied without any saline wash, has a similar property to the nitric ether; but as it is readily acted on by faint light, it is of greater value. However, papers prepared with it must be used within twenty-four hours, as they quickly lose their sensitiveness, and soon become nearly useless.

To fix with any degree of certainty the strength of the solution of the salts which will in all cases produce the best effects, appears impossible; every variety of light to which the paper has been exposed to darken, requiring a solution of different specific gravity.

In darkening these papers, the directions given at page 83 should be strictly followed.

Any of the following preparations may be employed as the bleaching agents.

Iodides Of Potassium And Sodium

The former of these salts being more easily procured than any other, is the one generally employed. The strength of the solution of these salts best adapted for the general kinds of paper is thirty grains to an ounce of water. The following results will exhibit the different energies manifested by these solutions at several strengths, as tried on the same paper by the same light:—

120 grains of the salt to an ounce of water took 12 minutes to whiten the paper.....

The other salts correspond nearly with these in their action ; a certain point of dilution being necessary with all.

Iodide Of Ammonia

Iodide Of Ammonia, if used on unsized paper, has some advantage as to quickness over the salts either of potash or of soda. This preparation is, however, so readily decomposed, that the size of the paper occasions a liberation of iodine, and the consequent formation of brown-spots.

Iodide Of Iron

This metallic iodide acts with rapidity on the darkened paper; but even in the shade its chemical energy is too great, destroying the sharpness of outline, and impairing the middle tints of the drawing. It also renders the paper very yellow.

Iodide Of Manganese

Iodide Of Manganese answers remarkably well when it can be procured absolutely free of iron. When the manganesic solution contains iron, even in the smallest quantities, light and dark spots are formed over the picture, which give it a curious speckled appearance.

Iodide Of Barium

Iodide Of Barium possesses advantages over every other simple iodine solution, both as regards quickness of action and the sharpness of outline. A solution may, however, be made still superior to it, by combining a portion of iron with it. Forty grains of the hydriodate of baryta being dissolved in one ounce of distilled water, five grains of very pure sulphate of iron should be added to it, and allowed to dissolve slowly. Sulphate of baryta is precipitated, which should be separated by filtration, when the solution is composed of iodide of barium and iron. By now adding a drop or two of diluted sulphuric acid, more baryta is precipitated, and a portion of hydriodic acid set free. The solution must be allowed to stand until it is clear and then carefully decanted off from the sediment, as filtering paper decomposes the acid, and free iodine is liberated. By this means we procure a photographic solution of a very active character. It should be prepared in small quantities, as it suffers decomposition under the influences of the atmosphere and light.

Hydriodic Acid

Hydriodic Acid, if used on paper which will not decompose it, acts very readily on the darkened silver. A portion of this acid, free in any of the solutions, most materially quickens the action. From the barytic solution it is always easy to set free the required portion, by precipitating the barytes by sulphuric acid. As the iodide of barium is rarely kept by the retail chemist, it may be useful to give an easy method of preparing the solution of the required strength.

Put into a Florence flask one ounce of iodine, and cover it with one fluid ounce and a half of distilled water ; to this add half a drachm of phosphorus cut into small pieces ; apply a very gentle heat until they unite, and the liquid becomes colourless ; then add another fluid ounce and a half of water. It is now a solution of hydriodic acid and phosphoric acid. By adding carbonate of barytes to it, a phosphate of barytes is formed, which, being insoluble, falls to the bottom, whilst the soluble iodide of barium remains dissolved. Make up the quantity of the solutions to nine ounces with distilled water, and carefully preserve it in a green glass stoppered bottle.