_Use of a solution of Oxide of Silver in Nitrate of Ammonia._--The great objection to the use of Ammonio-Nitrate of Silver is the _decomposition_ which it sometimes experiences by keeping, metallic Silver separating and Ammonia being set free. To obviate this liberation of Ammonia, the Author employs Nitrate of Ammonia as the solvent for the Oxide of Silver. The solution is prepared as follows:--Dissolve 60 grains of _Nitrate of Silver_ in half an ounce of water, and drop in Ammonia until the precipitated Oxide of Silver is exactly re-dissolved. Then divide this solution of Ammonio-Nitrate of Silver into two equal parts, to one of which add Nitric Acid cautiously, until a piece of immersed litmus-paper is reddened by an excess of the acid; then mix the two together, fill up to one ounce with water, and filter from the milky deposit of Chloride or Carbonate of Silver, if any be formed. This solution of Oxide of Silver in Nitrate of Ammonia appears to possess all the advantages of the Ammonio-Nitrate without the inconvenience of liberating so much free Ammonia upon the surface of the sensitive sheets. _Hints in selecting from the above Formulæ._--Albuminized paper is the most simple and generally useful; it is well fitted for small portraits and stereoscopic Photographs. The Ammonio-Nitrate Process requires more experience, but gives excellent results when black tones are required: it may be used for larger portraits, engravings, etc. Plain paper rendered sensitive by floating upon a Bath of Nitrate of Silver is easier of manipulation than the Ammonio-Nitrate, and will be found to be better adapted for toning by the Sel d'or Bath (p. 267) than the Albuminized Paper. PREPARATION OF THE FIXING AND TONING BATH. Take of Chloride of Gold 4 grains. Nitrate of Silver 16 grains. Hyposulphite of Soda[46] 4 ounces. Water 8 fluid ounces. [46] The common kind of Hyposulphite of Soda occurring in yellow and discoloured masses, is too impure for use in Photography, and requires re-crystallization. Dissolve the Hyposulphite of Soda in four ounces of the water, the Chloride of Gold in three ounces, the Nitrate of Silver in the remaining ounce; then pour the diluted Chloride by degrees into the Hyposulphite, stirring with a glass rod; and afterwards the Nitrate of Silver in the same way. This order of mixing the solutions is to be strictly observed: if it were reversed, the Hyposulphite of Soda being added to the Chloride of Gold, the result would be the reduction of Metallic Gold; Hyposulphite of Gold, which is formed, being an unstable substance, and not capable of existing in contact with unaltered Chloride of Gold. If however it be dissolved by Hyposulphite of Soda immediately on its formation, it is rendered more permanent, by conversion into a double salt of Soda and Gold.