It is important to remark in the solution of the insoluble Silver Salts by Alkaline Chlorides, Iodides, etc., that the amount dissolved is not in proportion to the _quantity_ of the solvent, but to the degree of concentration of its aqueous solution. This is not usual with solvents which act by entering into chemical combination with the substance dissolved. Commonly a given weight of the one salt dissolves a given weight of the other, independent of the amount of water present. The peculiarity in the case before us depends upon the fact that the double salt formed is _decomposed_ by a large quantity of water. Hence it is a _saturated_ solution of Chloride of Sodium which possesses the greatest power of fixing paper prints; and with the Bromide or Iodide of Potassium the same rule holds good--the stronger the solution the more Iodide of Silver will be taken up. The addition of water produces milkiness and a deposit of the silver Salt previously dissolved. ALKALINE HYPOSULPHITES. Hyposulphurous Acid is one of the Oxides of Sulphur. It is, as its name implies, of an acid nature, and takes its place upon the list immediately below Sulphurous Acid ("υρο," under). The Hyposulphite of Soda commonly employed by Photographers is a neutral combination of Hyposulphurous Acid and the alkali Soda. It is selected as being more economical in preparation than any other Hyposulphite adapted for fixing. Hyposulphite of Soda occurs in the form of large translucent groups of crystals, which include five atoms of water. These crystals are soluble in water almost to any extent, the solution being attended with the production of cold; they have a nauseous and bitter taste. In the solution of Silver compounds by Hyposulphite of Soda a _double decomposition_ always takes place; thus:-- Hyposulphite of Soda + Chloride of Silver = Hyposulphite of Silver + Chloride of Sodium. The Hyposulphite of Silver with an excess of Hyposulphite of Soda forms a soluble double salt, which may be crystallized out by evaporating the solution. It possesses an intensely sweet taste, and contains one atom of Hyposulphite of Silver, chemically combined with two of Hyposulphite of Soda. In addition to this there is a second double Salt, differing from the first in being _very sparingly_ soluble in water. It is formed by acting upon Chloride of Silver with a solution of Hyposulphite of Soda already saturated, or nearly so, with Silver Salts; and contains single atoms of each constituent. The fact that the Silver contained in an ordinary fixing Bath is present in the state of _Hyposulphite_ must be borne in mind, because this salt is liable to undergo peculiar chemical changes, as will be better shown in Chapter VIII.