The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Garcinia Morella, a native of Siam and Cochin China. The finest is that called " Pipe Gamboge," which is collected in bamboo canes. The inferior is called cake gamboge. It is almost entirely soluble in alcohol, and when rubbed down with water forms a thick emulsion of a brilliant yellow colour. It has but little taste, and no smell. It is used in photography as a colouring matter for varnishes and lacquers; also as a chemically opaque material in spotting negatives. Inks used by those who draw in line for process reproduction often contain gamboge. The action upon human beings when taken inter-nally is that of a drastic and hydragogue cathartic.
Gelatine. An animal substance obtained by boiling bones, hoofs, horns, and other animal substances. It contains about 15 to 20 per cent, of water at ordinary temperatures, and in cold water swells up and absorbs from five to ten times its weight. Good samples will absorb sufficient water to dissolve them when the temperature is raised above 900 F., the solution setting again to a jelly on cooling. The continued application of heat for some time destroys this setting powder, a modification called metagelatine being formed. Gelatine will keep indefinitely in the dry state, but in the presence of water it soon putrefies, turning first acid, and then alkaline; and at this stage ammoniacal vapours are given off. Alum, alcohol, carbolic, salicylic, and boracic acids, thymol, formalin, and the salts of zinc, act as antiseptics. Acetic, hydrochloric, sulphuric, and oxalic acids dissolve gelatine even in the cold - acetic acid very readily, and forming a useful liquid glue. Carbolic acid and alcohol pre-cipitate it from aqueous solutions when they are in excess. Silver nitrate exposed to sunlight in contact with gelatine pro-duces a red colour, due to a compound of gelatine and suboxide of silver. The alkaline bichromates in solution of gelatine render the latter after exposure to light insoluble and incapable of absorbing water, this action being the basis of the carbon and nearly every photo-mechanical printing process. Chrome alum and tannin render it insoluble, but capable of absorbing water. Ordinary alum raises the melting point, but does not render it completely insoluble. The composition of gelatine varies with the source from which it is obtained, but the following may be taken as the percentage composition : -