CAOUTCHOUC, the principal constituent of natural rubber (see RUBBER: BOTANY, CULTIVATION AND CHEMISTRY) and there fore sometimes called pure rubber. It occurs as a vegetable gum, mixed with from -5 to 8 times its own weight of other stances. Caoutchouc is a white resilient solid; at o—io° C it is hard and opaque, but it becomes soft and translucent above C. It contains carbon and hydrogen only, and belongs to the terpenes (q.v.). By polymerization (q.v.) isoprene can be converted to a product resembling caoutchouc but inferior to that synthesized by the plant. Mineral caoutchouc is one of the names of elaterite (q.v.), or elastic bitumen.