TRE CAOUTCHOUC GROUP. 263 



After twenty-four hours the Caoutchouc collects on the 

 surface, alum is added, it hardens, and is finally pressed. 

 It is said that old trees yield more than young ones, and 

 the juice flows more in cold than in hot weather. The 

 Caoutchouc from South America is brown or blackish ; 

 that from Madagascar, bluish or yellowish ; that from 

 ludia, white or yellowish ; and that from China, reddish. 

 The odor is peculiar, but not strong, and it has no taste. 

 It cannot be cut with a knife. It has a specific gravity of 

 .92 to .96, is a " non-conductor" of electricity, and insoluble 

 in water ; it swells up in alcohol, oil of turpentine, carbon 

 disulphide, ether, and benzine. It is not attacked by acids 

 at ordinary temperatures. Under the microscope it pre- 

 sents a homogeneous appearance, without any cavities. It 

 is used in the manufacture of optical and surgical instru- 

 ments ; for buttons, combs, stoppers, tubes ; for making 

 water-proof clothing, air-pillows, etc. Ebonite, which con- 

 tains more sulphur and is harder than ordinary Caoutchouc, 

 is used for those instruments and other objects for which 

 the latter would be too soft. 



2. Gutta-Percha is not so widely distributed as 

 Caoutchouc; it occurs in the milky juice of plants only 

 ■which belong to the family SapotacecB. The largest 

 quantity comes from Isoiiandra gutta, a tree of India and 

 the Sandwich Islands, usually three or four feet in diameter, 

 and sixty to eighty feet high. It has a straight trunk, 

 numerous ascending branches, with oblong, petiolate leaves, 

 crowded at their extremities. The flowers are small and 

 white; the wood is soft, fibrous, and of a pale color. 

 Gutta-Percha was known for a long time, and used by the 

 natives for handles for knives, weapons, etc. It was intro- 

 duced into Europe in 1843. The trees were felled, cut up. 



