_On certain states of the fixing and toning Bath which are injurious to the proofs._--The object of using the Hyposulphite Bath is to fix the proof and to tone it by means of Gold. But it is a fact familiar to the photographic chemist, that Positives can also be toned by a sulphuretting action, and that the colours so obtained are not very different from those which follow the employment of Gold.[20] Now the Hyposulphite of Soda is a substance which can be very readily made to yield up Sulphur to any bodies which possess an affinity for that element, and as the reduced Silver compound in the print has such an affinity, there is always a tendency to absorption of Sulphur when the proofs are immersed in the Bath. Consequently in many cases a sulphur toning-process is set up, and as the picture is improved by it in appearance, losing its brick-red colour and assuming a purple shade, it was at first adopted by Photographers. Experience however has shown that colours brightened in this way are less permanent than others, and are liable to fade unless kept perfectly dry. Hence the process will be discarded by all careful operators, and the object will be to avoid sulphuration as far as possible. This can be done to a great extent, and, when the Bath is properly managed, the prints will be toned almost entirely by Gold, and will, with care, be permanent. [20] For a more detailed account of the toning process by Sulphur, see the Third Section of this Chapter, page 145. The instability of sulphuretted prints is shown in the fourth Section. Some of the conditions which facilitate a sulphuretting action upon the proof are as follows:-- a. _The addition of an Acid to the Bath._--It was at one time common to add a few drops of Acetic Acid to the fixing Bath of Hyposulphite of Soda, immediately before immersing the proofs. The Bath then assumes an opalescent appearance in the course of a few minutes, and, when this milkiness is perceptible, the print begins to _tone_ rapidly and becomes nearly black. The chemical changes produced in a Hyposulphite Bath by addition of acid, may be explained thus:--The acid first displaces the feeble Hyposulphurous acid from its combination with Soda. Acetic Acid + Hyposulphite Soda. = Acetate Soda + Hyposulphurous Acid. Then the Hyposulphurous Acid, _not being a stable substance when isolated_, begins spontaneously to decompose, and splits up into Sulphurous Acid--which remains dissolved in the liquid, communicating the characteristic odour of burning Sulphur--and _Sulphur_, which separates in a finely divided state and forms a milky deposit.[21] [21] From the Vocabulary, Part III., it will be seen that commercial Chloride of Gold usually contains _free Hydrochloric Acid_; hence a considerable deposit of Sulphur takes place on adding it to the Hyposulphite solution, and the liquid must not be used immediately.