In fixing Photographic prints, this brown deposit of Sulphuret of Silver is very liable to form in the Bath and upon the picture; particularly so when the _temperature_ is high. To obviate it, observe the following directions:--It is especially in the reaction between _Nitrate of Silver_ and Hyposulphite of Soda that the blackening is seen; the Chloride and other _insoluble_ Salts of Silver being dissolved, even to saturation, without any decomposition of the Hyposulphite formed. Hence if the print be washed in water to remove the soluble Nitrate, a very much weaker fixing Bath than usual may be employed. But if the proofs are taken at once from the printing frame and immersed in a dilute Bath of Hyposulphite (one part of the salt to six or eight of water), _a shade of brown_ may often be observed to pass over the surface of the print, and a large deposit of Sulphuret of Silver soon forms as the result of the decomposition. On the other hand, with a strong Hyposulphite Bath there is little or no discoloration, and the black deposit is absent. The print must also be left for a sufficient time in the fixing bath, or some appearance of brown patches,[18] visible by transmitted light, may occur. Each atom of Nitrate of Silver requires _three_ atoms of Hyposulphite of Soda to form the _sweet and soluble double salt_, and hence, if the action be not continued sufficiently long, another compound will be formed almost tasteless and insoluble (p. 44). Even immersion in a new Bath of Hyposulphite of Soda does not fix the print when once the yellow stage of decomposition has been established. This yellow salt is insoluble in Hyposulphite of Soda, and consequently remains in the paper. [18] The writer has noticed that when sensitive paper is _kept for some time_ before being used for printing, these yellow patches of imperfect fixation are very liable to occur. The Nitrate of Silver appears gradually to enter into combination with the organic matter of the size of the paper, and cannot then be so easily extracted by the fixing bath. In fixing prints by Ammonia the Author has found that the same rule may be applied as in the case of Hyposulphite of Soda, viz. that if the process be not properly performed, the white parts of the print will appear _spotted_ when held up to the light, from a portion of insoluble Silver Salt remaining in the paper. Prints imperfectly fixed by Ammonia are also usually brown and discoloured upon the surface of the paper. More exact directions as to the strength of the fixing bath and the time occupied in the process, will be given in the Second Part of the Work; at present it may be noticed only that _Albuminized_ paper, from the horny nature of its surface-coating, requires a longer treatment with the Hyposulphite than the plain paper. THE SALTS OF GOLD AS TONING AGENTS FOR PHOTOGRAPHIC PRINTS.