Barium Bromide Barium Iodide
together with Canada balsam. Occasionally these cemented surfaces separate more or less completely, or the balsam under-goes a kind of congelation ; while occasionally it becomes so yellow as to make the lens extremely slow in its action. The separation, cleaning, and recementing which then becomes necessary is neither difficult nor does it involve much risk, although, where practicable, it is better to send the lens to the maker. The principle points of the operation, as described by Mr. J. Trail Taylor, are: - (i) Boiling the combination in water, with precautions against mechanical damage and sudden heating ; (2) separation by sliding one glass over the other; (3) clearing by ether, old collodion, or alcohol; (4) recementing. For this purpose the lens with the concavity is laid on a sheet of paper and a large drop of clear Canada balsam is placed centrally on the concavity. The convex surface is now brought down steadily into its place, so that the balsam exudes all round. The two glasses are now tied with a long string, which is made to cross and recross in many directions, care being taken that the edges of the glasses coincide. In this condition the lens is allowed to remain in a warm place until the balsam at the edges has hardened, when the string is removed and the lens cleaned by similar means to those previously employed.
Barium Bromide (Ger., Baryum bromid; Fr., Bromure de baryum; Ital., Bromuro di barid). BaBr2. Crystallises with 2H,0. Solubility : 100 parts are soluble in 96 parts of cold water, and in 75 parts boiling; it is also soluble in alcohol and ether. Occasionally used in collodion. Like all barium salts it is poisonous.
Barium Chloride (Ger., Bariumchlorid or Chlorbarium ; Fr., Chlorure de baryum; Ital., Cloruro di bario). BaCl2. Occurs as white tabular crystals, permanent in air, which lose their water of crystallisation at 1500. It is poisonous. It is occasionally employed in salting albumenised paper, and for making Baryta Paper (q.v.) and imitation opal glass. Solubility: 36 per cent, in cold, 59 per cent, in boiling, water; 001 per cent, in cold alcohol, 0-5 per cent, in hot alcohol, 10 per cent, in glycerine.
Barium Iodide (Ger., Iodbarium; Fr., Iodure de baryum; Ital., Ioduro di bario). Bal2 = 391. Deliquescent tabular