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60 THE ADVANCE OF PHOTOGRAPHY
in the industrial arts, which possesses the property of powerfully attracting the oxygen of the air, or rusting, so that it has to be protected by being kept under naphtha. The chlorides, bromides, and iodides of the metals all show a nature analogous to salt. Chloride, bromide, and iodide of silver are particularly interesting to us. These three salts may be obtained by the direct action of chlorine, bromine, and iodine on silver ; but a more rapid method is to dissolve in water chloride, bromide, or iodide of sodium and to add to them a solution of a salt of silver.
If a silver coin is thrown into nitric acid, it is dissolved, forming nitrate of silver ; and this is obtained on evaporating the solution as a white soluble salt, which when fused is called lunar caustic.
If a solution of this substance be mixed with a solution of chloride of sodium, a white curdy precipitate of chloride of silver is formed by double decomposition. Chloride of sodium and nitrate of silver produce chloride of silver and nitrate of sodium.
Bromide or iodide of silver may be produced exactly in the same manner if bromide or iodide of sodium be added to a solution of silver nitrate.
Bromide, chloride, and iodide of silver are thus separated as precipitates, because they are all three insoluble in water. After being washed and dried, all three salts appear in the form of powders, the chloride being white, the bromide yellowish white, and the iodide yellow. All three are very stable bodies, not decomposed by heat, and insoluble in water, alcohol, or ether ; they are, however, dissolved by solutions of hypo-sulphite of soda or cyanide of potassium, by combining with these bodies to form new chemical compounds which are soluble in water.
These three stable compounds — chloride, bromide, and iodide of silver — show a marked sensitiveness to light, and this sensitiveness is the basis of modern photography.