BROMIDE OF POTASSIUM. B. Br. When pure, this is the most convenient bromide to select, when a sensitive surface containing bromide of silver is to be formed by double decomposition. Its usual impurities are carbonate of potassa and chlmide of potassium, derived from the carbonate used in its manufactine. The first of these, from its alkaline nature, will be injurious if in great excess ; but it is easy to get rid of it by adding bromine to the aqueous solution of the bromide, until a little c,olour is produced, and after wards evaporating and fusing. Bromide of potassium has been of service on some occasions, in partially fixing negative and positive proofs, which it does by forming, with the unreduced and insoluble silver salt, a soluble double salt of silver and potassium. Other bromides, and many iodides, chlorides, and cyanides, act in the same way, and even nitrate of silver. These double salts are all
partially decomposed by the smallest dilution of their concentrated solutions with reprecipitation of the insoluble silver salt, and their solvent power increases wonderfully with their degree of concentra tion. They must, consequently, be used in large quantities, when water is to be afterwards freely used in their removal. Hypo sulphite of soda comes under the same rule, and caimot be advan tageously had recourse to except this law be remembered. The principal use of this and the other bromides (see " Bromide "), in photography, has been, in the collodion and paper processes, to form a mixture of bromide of silver with iodide in the sensitive surfaces, and it has been occasionally employed alone, without any iodide. For the advantages sought for in this mixture, 8ee "Bromide of Silver," in the next article.