| Sulphur. | Oxygen. | Formulæ. | |||
| Dithionic or Hyposulphuric Acid | 2 | atoms | 5 | atoms | S2O5 |
| Trithionic Acid | 3 | " | 5 | " | S3O5 |
| Tetrathionic Acid | 4 | " | 5 | " | S4O5 |
| Pentathionic Acid | 5 | " | 5 | " | S5O5 |
The amount of Oxygen in all is the same, that of the other element increases progressively; hence it is at once evident that the highest member of the series might by losing Sulphur descend gradually until it reached the condition of the lowest.
This transition is not only theoretically possible, but there is an actual tendency to it, all the acids being unstable with the exception of the Hyposulphuric. The Alkaline Salts of these acids are more unstable than the acids themselves; a solution of Tetrathionate of Soda becomes milky in the course of a few days from deposition of Sulphur, and, if tested, is then found to contain Trithionate and eventually Dithionate of Soda.
The cause of the change in properties of the fixing Bath being thus clearly traced to a decomposition of Hyposulphite of Silver, and a consequent generation of unstable principles capable of imparting Sulphur to the immersed proofs, it seemed desirable to continue the experiments.—
There is a peculiar acid condition commonly assumed by old fixing Baths, which could not be satisfactorily explained, since it was known that acids do not exist long in a free state in solution of Hyposulphite of Soda, but tend to neutralize themselves by displacing Hyposulphurous Acid spontaneously decomposable into Sulphurous Acid and Sulphur. This point is set at rest by the discovery of a peculiar reaction which takes place between certain salts of the Polythionic acids and Hyposulphite of Soda. A solution of Tetrathionate of Soda may be preserved for many hours unchanged; but if a few crystals of Hyposulphite of Soda be dropped in, it begins very shortly to deposit Sulphur, and continues to do so for several days. At the same time the liquid acquires an acid reaction to test-paper, and produces effervescence on the addition of Carbonate of Lime.
It is evident that a Sulphur acid exists which has not hitherto been described, and that this acid is formed as one of the products of the decomposition of the Hyposulphite of Silver contained in the fixing Bath. The subject is an important one to Photographers, because it is found that Hyposulphite Baths which have acquired the acid reaction, although toning quickly, yield Positives which fade on keeping. The acid may perhaps combine with the reduced Silver Salt, which, if the image be allowed to contain Suboxide of Silver, is theoretically probable.
The experiments were next directed towards ascertaining more carefully the effect of the acid fixing Bath upon the Positive proofs. Tetrathionate of Soda added to solution of Hyposulphite of Soda produces, at the expiration of twelve hours, a liquid which, when filtered from the deposited Sulphur, reddens blue litmus-paper slowly. Positive prints immersed in the Bath pass from red to black, dissolving in the half-tones, and becoming yellow and faded if the action be too long continued. On adding Carbonate of Soda in quantity sufficient to remove the acid reaction, the power of toning is much diminished, but dark colours can still be obtained by continuing the action. The solvent effect upon the half-tones, evidently caused in great measure by the acid, is lessened; whilst the tendency to yellowness in the white parts of the proof, almost disappears. These effects are more particularly manifested when the prints are immersed in the Bath immediately on their removal from the printing frame; and it is found almost impossible to preserve the whites of the impression clear, in the acid Bath, unless the Nitrate of Silver has been washed away.
Solution of half-tones and yellowness in the lights, both a source of annoyance to the operator, are thus traced in great measure to an acid condition of the fixing and toning Bath; and the remedy is obvious.
The Author's experiments upon the Tetrathionates and their reaction with Hyposulphite of Soda likewise elicited the important fact that alkalies decompose the unstable sulphuretted principle. If the Bath be treated with Potash or Carbonate of Soda, an alkaline Sulphuret appears to be gradually formed, which precipitates Sulphuret of Silver, and in the course of a few days the liquid returns to its original condition and ceases to act as a toning agent upon the proof. The same effect takes place to a great extent when the solution is set aside for several weeks or months; a process of spontaneous change going forward, which issues in a deposition of Sulphur and Sulphuret of Silver, and a partial loss of sulphuretting properties in the liquid.
It may be interesting to the scientific investigator to describe the mode of preparing a fixing and toning Bath, illustrating the above remarks:—