GAMBOGE. A gum resin, concreted from the milky juice which exudes from the Gambogia gutta, and Guttafera vera, trees native in Ceylon and Siam. It consists of about 85 parts of a red resin soluble in alcohol, and 15 parts of gum. It is a valuable pigment, and may be used for painting out the skies of negatives.
GELATINE. This substance is produced by the action of hot water on the membranous tissues of animals. • To obtain it, such substances as clippings of hides, hoofs, horns, calves' feet, cow's heel, sheep's trotters, pig's pettitoes, certain membranes, &c., are cleansed in cold water and then boiled. The solution so ob tained is freed from fat, and any deposit, by skimming and straining, and allowed to form a jelly on cooling. This is called size, and when cut into slices and dried is called glue.
The purest form of gelatine is isinglass, which is obtained in Russia from the air bladder and sound of a species of sturgeon.
Size is sometimes obtained from the waste of vellum, parchment, and some kinds of white leather, and also from bones. It may be rendered inodorous, tasteless, and colourless, by the careful applica tion of sulphurous acid. It is then called patent gelatine, greme tine, &c.
Gelatine gradually softens and swells in cold water hut does not dissolve without heat. It absorbs three or four times its weight of cold water. 1 part of isinglass dissolved in 100 of hot water gelatinizes on cooling, but in 150 parts remains liquid : the effect, however, varies with the temperature.
When a solution of gelatine is repeatedly boiled and cooled it loses its power of gelatinizing on cooling, and remains soluble and delique scent. In this state it has been called by photographers metagelatine.
Gelatine is insoluble in absolute alcohol, and ether, and also in fixed and volatile oils. When alcohol is added to a warm and strong aqueous solution of gelatine, the gelatine separates as a white viscid substance ; and if a drop of the same solution of gelatine be added to alcohol, ether, or collodion, the gelatine immediately rolls itself up into a white ball, and sinks to the bottom of the bottle.
Gelatine is soluble in all the dilute acids, differing essentially in this respect from albumen. Of these, the acetic solution only gela tinizes on evaporation.
The dilute caustic alkalis, and ammonia, do not prevent the gela tinization of gelatine, but often throw down a portion of phosphate of lime. When gelatine is dissolved in a cold dilute solution of caustic potass, and exactly neutralized with acetic acid, the evapor ated liquor does not gelatinize on cooling ; it leaves a residue of altered gelatine combined with acetate of potass, which is soluble in alcohol. This substance might in certain cases be added to collodion to increase the density of the negative ; or it might be used as a preservative solution to be applied to a sensitive plate.
Tannin precipitates gelatine from its solution as a dense white curdy precipitate, called tanno-gelatine. It is on this principle that leather is produced by the long soaking of hides in infusion of oak bark. Tannin is a very delicate test of gelatine, for when added to a solution of 1 part of gelatine in 5000 of water a cloudiness is pro duced. Sulphate of platina produces the same effect, and is the better test of the two, as it does not act in a similar way on albumen, as tannin does. A mixture of salt and alum also forms a white precipitate with gelatine.
Gelatine is capable of combining with some of the metallic oxides ; for instance, with the oxides of iron, chromium, lead, tin, mercury, silver, gold, and platinum, its combination with silver and gold being effected by means of light, for it does not readily alter the colour of their solutions, if kept in the dark.
Gelatine has many important uses in photography, and enters largely into the sizing of English photographic papers.