The silver bath is also contaminated with organic matter, taken up from the edges and backs of the many plates used, of which some portion of the albumen is likely to be exposed to the action of the solution, and also from dust, etc., falling into the bath dish, which will be taken up and held in solution by the acid in the bath, but when the acid can hold no more in solution, its presence will be indicated by a greyish white vail forming on the surface of the plate when developed; this is called fog, and may only partially obscure the image, or it may completely veil it.

The fogging of the plates in this manner may be prevented for a time by adding more nitric acid to the bath, which will dissolve the organic matter and prevent its deposition on the plate.

When the silver bath has, from long or much use, become seriously affected by any of these evils, it should be set aside and a fresh bath put in use, until an opportunity offers to thoroughly cleanse and renew the old solution, which can be done by one series of manipulations, as thus :

Into a large bottle, which will hold the solution to be treated, and somewhat more, put from a pint to a quart of water, according to the quantity of the solution; into this bottle containing the pint of water, pour in a small stream the whole of the old solution, when it will be found to have turned to a milky whiteness or opalescence, which is the iodide of silver set free by reducing the strength of the solution; this must be filtered out, and the solution neutralized with bi-carbonate of soda, which should be put in, in small quantities until a deposit of carbonate of silver begins to be formed; then place the solution in an evaporating dish, and subject to heat until the bulk is reduced about one-half; test it with litmus paper, and if acid, give it more bi-carbonate of soda; add water and silver nitrate to bring to the required bulk and strength, set in the sunlight for a time or until required for use, then filter, and it will be found to be as good as new, but it may require a few drops of nitric acid.

Pure water may be always had ready for use, by keeping a large bottle full of the common water standing out-door in the sunlight, adding a grain of silver from time to time as the bottle is emptied and replenished.

The new "agate iron ware" pots are the best for use as evaporating dishes. They will not break or crack, and they are not nearly so expensive as porcelain dishes.