The occasional addition of fresh crystals of Hyposulphite of Soda to keep up the strength of the Bath, is useful, the exact quantity added not being material. _The washing, drying, and mounting of the Positive Proofs._--It is essential to wash out every trace of Hyposulphite of Soda from the Print if it is to be preserved from fading, and to do this properly requires considerable care. Always wash with running water when it can be obtained, and choose a large shallow vessel exposing a considerable surface in preference to one of lesser diameter. A constant dribbling of water must be maintained for four or five hours, and the prints should not lie together too closely, or the water does not find its way between them, (see the remarks at p. 162). When running water cannot be obtained, proceed as follows:--first wash the Prints gently, to remove the greater part of the Hyposulphite solution. Then transfer them to a large shallow pan, in which may be placed as many Prints as it will conveniently hold. Leave them in for about a quarter of an hour, with occasional movement, and then pour off the water quite dry. This point is important, viz. to drain off the last portion of liquid completely before adding fresh water. Repeat the process of changing at least five or six times, or more, according to the bulk of water, number of Prints, and degree of attention paid to them. Lastly, proceed to remove the size from the Print by immersion in boiling water.[48] This process will give some idea of the permanency of the tints, since, if they become dull and red, _and do not darken on drying_, the Print is probably toned without Gold. Ammonio-Nitrate and plain paper Prints prepared on foreign papers by the modes described in this Work, may be expected to stand the test of boiling water; Albumen Prints and Positives on English paper are a little reddened, although not to an objectionable degree. [48] The Print must be well washed in cold water, to remove the Hyposulphite, before using the hot water; or the half-tones will be liable to be darkened, or changed to incipient yellowness, by sulphuration. This point is important as regards the permanency.