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 odour, 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. TETRATHIONIC ACID. Symbol, S{4}O{5}. Atomic weight, 104. The chemistry of the Polythionic Acids and their salts will be found described in the First Part of this Work, page 157. WATER. Symbol, HO. 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 discolours Nitrate of Silver, and is therefore injurious. _Rain-water_, having undergone a natural process of distillation, is free from inorganic salts, but it usually contains a minute portion of _Ammonia_, which gives it an alkaline reaction to test-paper. It is very good for Photographic purposes if collected in clean vessels, but when taken from a common rain-water tank should always be examined, and if much organic matter be present, tingeing it of a brown colour and imparting an unpleasant smell, it must be rejected. _Spring_ or _River_ water, commonly known as "hard water," usually contains Sulphate of Lime, and Carbonate of Lime dissolved in Carbonic Acid; also Chloride of Sodium in greater or less quantity. On boiling the water, the Carbonic Acid gas is evolved, and the greater part of the Carbonate of Lime (if any is present) deposits, forming an earthy incrustation on the boiler. In testing water for Sulphates and Chlorides, acidify a portion with a few drops of _pure_ Nitric Acid, free from Chlorine (if this is not at hand, use pure Acetic Acid); then divide it into two parts, and add to the first a _dilute_ solution of Chloride of Barium, and to the second, Nitrate of Silver,--a milkiness indicates the presence of Sulphates in the first case or of Chlorides in the second. The _Photographic Nitrate Bath_ cannot be used as a test, since the Iodide of Silver it contains is precipitated on dilution, giving a milkiness which might be mistaken for Chloride of Silver.