"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 meanwhile with a glass rod; and afterwards the nitrate of silver in the same way. This order of mixing the solution 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. The difference depends upon the fact that the hyposulphite of gold which is formed is an exceedingly unstable substance, and cannot exist in contact with unaltered chloride of gold. It is necessary that it should be dissolved by hyposulphite of soda _immediately_ on its formation, and so rendered more permanent by conversion into a double salt of soda and gold. "The _time of coloration_ depends much upon the quantity of gold present, and may in some cases be extended to many hours. The results of a few experiments, performed roughly, appeared to indicate that the activity of this bath is less affected by depression of temperature than those prepared with tetrathionate. Certainly the injurious effects of prolonged immersion are not so evident as with the first two formulæ: the purity of the whites remains unaltered for many hours if the bath is new, but with an old bath there is a tendency to yellowness, which is probably caused by the presence of sulphuretted hydrogen. Fresh chloride of gold must be added from time to time, as it appears to be required." After the impression has remained in the toning bath a sufficient length of time, it should be placed in a dish or sink of clean water, which should be changed several times--floating for at least 12 hours; then it may be taken out and hung up to dry. "Touching."--The _coloring_ of a photograph forms no part of my process: this is a matter to be given into the hands of an artist, and when it bears the finishing touch of his skill, it is no longer a _photograph_, but _an oil_ or _watercolor painting_; all the delicate workings of nature having been lost or hidden under the colors. A photograph may often be "touched" to advantage. If, as is frequently the case, there be little white spots on the face of the paper, they may be readily covered by the application of a little India ink, with the point of a wet pencil or fine small brush. Mounting of Positives. This, though a small matter in itself, is worthy of great attention. The durability of the proof depends much upon the purity of the paste used in causing its adhesion to the Bristol board. I have employed the following composition with the most eminent success:--