Reservoir
Dams constructed for the purpose of gathering a large quantity of metal are sometimes called reservoirs; as also are runner-basins when constructed of extraordinary dimensions to receive the molten metal from very large pouring-ladles. See DAMS; BASIN; GATHERING-METAL. Resin is a solid, inflammable substance, of vegetable origin, being obtained from various trees by making incisions in their bark and allowing the liquid to exude. This liquid is the essential oil of the plant, and holds the resin in solution. Resins are insoluble in water, but alcohol dissolves them; they are of an inflammable nature, and yield a dense, sooty smoke when burning. Mastic, sandarac, lac, copal, etc., are some of the resins from which varnishes are made, all of which are readily dissolved in such solvents as spirits of wine, oil of turpentine, methylated spirit, and wood naphtha. The evaporation of the spirit, after these varnishes have been applied, leaves a hard layer of the resin on the surface of the object treated. The common resin, or rosin, of commerce is obtained from the various species of pine. Gum-resins are the solidified milky exudations of plants. They consist of resin, essential oils, and a gummy substance peculiar to the plant. The ammoniacum, assafoetida, aloes, myrrh, gamboge, etc., belong to this class; they are all soluble in rectified alcohol, and are valuable as medicinal agents principally. Elastic gums, as caoutchouc or india-rubber and gutta-percha, are valuable in the arts and manufactures; the former consists of a thick milky juice of certain trees growing in tropical countries, and is a mixture of several hydrocarbons with turpentine oil. When pure it is nearly white. It will soften in boiling water, but not dissolve; it is also insoluble in alcohol, but readily dissolves in coal naphtha, rectified oil of turpentine, pure ether, chloroform, or carbonic disulphide. Caoutchouc is rendered more permanently elastic by combining with it certain proportions of sulphur. It is then called vulcanized india-rubber. See GUTTA-PERCHA; INDIA-RUBBER.