It appeared therefore probable, upon theoretical grounds, that the Penta-, Tetra-, and Trithionates might produce some effect in the Hyposulphite fixing Bath. Upon making the trial these expectations were verified; and it was found that Tetrathionate of Soda added to Hyposulphite of Soda yielded a fixing and toning Bath quite equal in activity to that produced by means of Chloride of Gold. It may be useful to review for an instant the composition of the Polythionic series of acids; it is thus represented:-- Sulphur. Oxygen. Formulæ. Dithionic or Hyposulphuric Acid 2 atoms 5 atoms S{2}O{5} Trithionic Acid 3 " 5 " S{3}O{5} Tetrathionic Acid 4 " 5 " S{4}O{5} Pentathionic Acid 5 " 5 " S{5}O{5} 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 _Tri_thionate and eventually _Di_thionate 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.