In order to understand, more fully how _decomposition_ of hyposulphite of silver may affect the process of fixing, the peculiar properties of this salt should be studied. With this view nitrate of silver and hyposulphite of soda may be mixed in equivalent proportions, viz. about twenty-one grains of the former salt to sixteen grains of the latter, first dissolving each in separate vessels in half an ounce of distilled water. These solutions are to be added to each other and well agitated; immediately a dense deposit forms, which is hyposulphite of silver. At this point a curious series of changes commences. The precipitate, at first white and curdy, soon alters in color: it becomes canary-yellow, then of a rich orange-yellow, afterwards liver-color, and finally black. The _rationale_ of these changes is explained to a certain extent by studying the composition of the hyposulphite of silver. The formula for this substance is as follows:-- AgO S{2}O{2}, But AgO S{2}O{2} plainly equals AgS, or sulphuret of silver, and SO{3}, or sulphuric acid. The acid reaction assumed by the supernatant liquid is due therefore to sulphuric acid, and the black substance formed is sulphuret of silver. The yellow and orange-yellow compounds are earlier stages of the decomposition, but their exact nature is uncertain. The instability of hyposulphite of silver is principally seen when, it is in an isolated state: the presence of an excess of hyposulphite of soda renders it more permanent, by forming a double salt. 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 first 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. This plan, however, involving a little additional trouble, is, on that account, often objected to, and, when such is the case, a _concentrated_ solution of hyposulphite of soda must be used, in order to dissolve off the white hyposulphite of silver before it begins to decompose. When 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 this decomposition. On the other hand, with a strong hyposulphite bath there is little or no discoloration and the black deposit is absent.