_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 maybe 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 know a as Nitro-Sulphuric Acid. Sulphuric Acid possesses intense chemical powers, and displaces the greater number of ordinary acids from their salts. It _chars_ organic substances, by removing the elements of water, and converts Alcohol into Ether in a similar manner. The _strength_ of a given sample of Sulphuric Acid may be calculated, nearly, from its specific gravity, and a Table is given by Dr. Ure for that purpose. (See Appendix.) _Impurities of Commercial Sulphuric Acid._--The liquid acid sold as Oil of Vitriol is tolerably constant in composition, and seems to be as well adapted for Photographic use as the _pure_ Sulphuric Acid, which is far more expensive. The specific gravity should be about 1·836 at 60°. If a drop, evaporated upon Platinum-foil, gives a fixed residue, probably Bisulphate of Potash is present. A milkiness, on dilution, indicates Sulphate of Lead (see page 186). _Test for Sulphuric Acid._--If the presence of Sulphuric Acid, or a soluble Sulphate, be suspected in any liquid, it is tested for by adding a few drops of dilute solution of Chloride of Barium, or Nitrate of Baryta. A white precipitate, _insoluble in Nitric Acid_, indicates Sulphuric Acid. If the liquid to be tested is very acid, from Nitric or Hydrochloric Acid, it must be largely diluted before testing, or a crystalline precipitate will form, caused by the sparing solubility of the Chloride of Barium itself in acid solutions. SULPHUROUS ACID. Symbol, SO{2}. Atomic weight, 32. This is a gaseous compound, formed by burning Sulphur in atmospheric air or Oxygen gas: also by heating Oil of Vitriol in contact with metallic Copper, or with Charcoal.