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 generally be calculated from its specific gravity, and a table is given by Dr. Ure for that purpose. _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. _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. When an acid of any kind is added to hyposulphite of soda, sulphurous acid is formed as a product of the decomposition of hyposulphurous acid, but it afterwards disappears from the liquid by a secondary reaction, resulting in the production of trithionate and tetrathionate of soda. _Properties._--Sulphurous acid possesses a peculiar and suffocating odor, familiar to all in the fumes of burning sulphur. It is a feeble acid, and escapes with effervescence, like carbonic acid, when its salts are treated with oil of vitriol. It is soluble in water. _Water._ Symbol, H{2}O. Atomic weight, 9. Water is an oxide of hydrogen, containing single atoms of each of the gases. _Distilled water_ is water which has been vaporized and again condensed: by this means it is freed from earthy and saline impurities, which, not being volatile, are left in the body of the retort. _Pure_ distilled water leaves no residue on evaporation, and should remain perfectly clear on the addition of nitrate of silver, _even when exposed to the light_; it should also be neutral to test-paper. The condensed water of steam-boilers sold as distilled water is apt to be contaminated with oily and empyreumatic matter, which discolors nitrate of silver, and is therefore injurious.