If not sufficiently pure, repeat the crystallization until it is so.

For gilding, dissolve in water and use in the same manner as the common gilding solution.
N.B .-- The four following mixtures were employed in Neipce's process in his earliest experiments: Aqueous Solution of Bichloride of Mercury .-- Eight grains of bichloride of mercury in 10,000 grains of distilled water.
Solution of Cyanide of Mercury .-- A flask of distilled water is saturated with cyanide of mercury, and a certain quantity is decanted, which is diluted with an equal quantity of distilled water.
Acidulated White Oil of Petroleum .-- This oil is acidulated by mixing with it one tenth of pure nitric acid, leaving it for at least 48 hours, occasionally agitating the flask.

The oil, which is acidulated, and which then powerfully reddens litmus paper, is decanted.

It is also a little colored, but remains very limpid.
Solution of Chloride of Gold and Platinum .-- In order not to multiply the solutions, take the ordinary chloride of gold, used for fixing the impressions, and which is composed of 1 gramme of chloride of gold and 50 grains of hyposulphate of soda, to a quart of distilled water.
With respect to chloride of platinum, 4 grains must be dissolved in 3 quarts of distilled water; these two solutions are mixed in equal quantities.
Acids .-- I shall not go into the preparations of the various acids employed in the Daguerreotype.

This would be useless to the operator, as there are few, if any, that it would be advisable to prepare.