BROMIDES. The most useful salts of this class have been specified above. In the selection of the most suitable for the purpose in hand, attention must be paid especially, 1st, to their freedom from carbonates and oxides ; 2nd, to their stability in crystals and in their proper solutions ; 3rd, to the nature of the nitrate which they produce in the N. S. bath ; 4th, to their degree of solubility in ether and alcohol, when they are employed in collodion. The bromides of the metals cadmium, zinc, &c., are generally the purest and most stable salts ; those of the alkaline and earthy bases are more apt to contain alkaline matters. Bromide of potassium is not easily soluble in ether and alcohol, and therefore in collodion is apt to be partially precipitated in fine particles, which produce trans parent circular spots in the picture, and to produce, by its little solubility, a film of a misty appearance insensitive to light. The nitrates of the alkalis introduced by their bromides into the exciting bath are perfectly neutral, but those of the metals are slightly acid, though not injuriously. The nitrate of zinc is deliquescent, and helps, when formed in the collodion film, to keep it moist. Nitrate of ammonia is often prejudicial, and therefore, bromide of ammonium, a very unstable compound in collodion, is generally to be avoided.
Bromide of iron is also prejuclicial, from the decomposition which the nitrate of iron sets up in the nitrate of silver, on the film and in the bath. The choice lies, for most purposes, between the salts of potassium and cadmium.
BnomrNE. (Bpolioc a stench). A deep reddish brown liquid of a disagreeable odour. In its chemical relations it resembles oxygen, but is a less powerful body, and therefore many bromides will at common, and still more so at high temperattuts, absorb oxygen. The displaced bromine then partly unites with the salt, t,o form bibromide and oxibromide of a red colour, and partly escapes. Its vapour is extremely poisonous ; a single drop of the liquid on the beak of a bird, instantly kills it. Its aqueous solution is used to give off bromine vapour in the daguerreotype process, but is not so good for this purpose as the bromide of lime. Bromine has been added to collodion, but it has no advantage over iodine, since the effect of both is the same, viz., to liberate nitric acid in the bath and film.