Dissolve the Gold and Hyposulphite of Soda each in two ounces of the water; then mix quickly by pouring the former solution into the latter, and add the Hydrochloric Acid. If the Chloride of Gold be neutral, the liquid will have a red tinge, but if _acid_, then the solution may be colourless. The commercial Chloride of Gold, containing usually much free Hydrochloric Acid, will not require any addition of that substance. (See the Vocabulary, Part III.) In place of making an extemporaneous Hyposulphite of Gold by mixing the Chloride with Hyposulphite of Soda, the Crystallized Sel d'or may be used, adding about half a grain to the ounce of water, acidified as before; but the objection to the employment of this salt is its expense, and also the difficulty of obtaining it in a pure form; some samples containing less than five per cent, of Gold. It will be found very convenient to keep the two solutions on hand ready for mixing, viz. the Chloride of Gold dissolved in water in the proportion of a grain to the drachm, and the Hyposulphite of Soda, three grains to the drachm. When required for use, measure out a fluid drachm of each, dilute with water to two ounces, and mix. It is possible that the three-grain solution of Hyposulphite of Soda may by long keeping become decomposed, with precipitation of Sulphur. The effect of this would be to produce a turbidity and deposit of Gold on mixing the ingredients for the Bath, the Chloride of Gold being in excess over the Hyposulphite of Soda (see p. 250). The Bath of Sel d'or is always most active when recently mixed, but it will keep good for some days if contact with free Nitrate of Silver be avoided. The addition of this substance produces a red deposit in the Bath, containing Gold, and the solution then becomes useless. DETAILS OF MANIPULATION. The paper may be prepared by either of the formulæ given in the first Section of this Chapter, according to the tint desired. The pure black tones are obtained most easily with the Ammonio-Nitrate paper, and the purple tints, without gloss, on paper prepared with plain Chloride and Citrate of Soda. The printing is not carried quite to the usual intensity, as the half-tones are very little dissolved in this process. On being taken from the frame, the prints are washed thoroughly in common water until it ceases to become milky; that is, until the greater part of the Nitrate of Silver has been removed. The washing must be conducted in a dark place, but it is not necessary to hasten it; the proofs may be thrown into a pan of water covered with a cloth, and allowed to remain until required for tinting. A trace of free Nitrate of Silver usually escapes the washing; this would cause a yellow deposit on the Print, and also in the toning Bath. It must therefore be removed, either by adding a little _common salt_ to the water during the last washings, or by means of a dilute solution of Ammonia.