But even if, by a preliminary removal of the nitrate of silver, the danger of blackening be in a great measure obviated, yet the print must not be taken out of the fixing bath too speedily, or some appearance of brown patches, 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. 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. Sugar of Milk. (_See_ Milk.) Sulphuretted Hydrogen. (_See_ Hydrosulphuric Acid.) Sulphuric Acid. Symbol, SO{3}. Atomic weight, 40. Sulphuric acid maybe formed by oxidizing sulphur with boiling nitric acid; but this plan would be too expensive to be adopted on a large scale. The commercial process for the manufacture of sulphuric acid is exceedingly ingenious and beautiful, but it involves reactions which are too complicated to admit of a superficial explanation. The sulphur is first burnt into gaseous sulphurous acid (SO{2}), and then, by the agency of binoxide of nitrogen gas, an additional atom of oxygen is imparted from the atmosphere, so as to convert the SO{2} into SO{3}, or sulphuric acid. _Properties._--Anhydrous sulphuric acid is a white crystalline solid. The strongest liquid acid always contains one atom of water, which is closely associated with it, and cannot be driven off by the application of heat. This _mono-hydrated_ sulphuric acid, represented by the formula HO SO{3}, is a dense fluid, having a specific gravity of about 1·845; boils at 620°, and distils without decomposition. It is not volatile at common temperatures, and therefore does not _fume_ in the same manner as nitric or hydrochloric acid. The concentrated acid may be cooled down even to zero without solidifying; but a weaker compound, containing twice the quantity of water, and termed _glacial_ sulphuric acid, crystallizes at 40° Fahr. Sulphuric acid is intensely acid and caustic, but it does not destroy the skin or dissolve metals so readily as nitric acid. It has an energetic attraction for water, and when the two are mixed, condensation ensues, and much heat is evolved; four parts of acid and one of water produce a temperature equal to that of boiling water. Mixed with aqueous nitric acid, it forms the compound known as nitro-sulphuric acid.