CAOUTCHOUC. 387 



It is also used as a solvent of sulphur, phosphorus, fats, 

 resins, and caoutchouc. Both asphaltum and petroleum 

 have been, probably, produced by the action of vol- 

 canic fires upon bituminous coal. 



970. GUM RESINS. The dried juices 



what is said . , 



of gum re- of certain plants consist of mixtures of 

 gum and resin. These mixtures are called 

 gum resins. Water dissolves the gum, and holds the 

 resin in suspension, thus forming what is called an 

 emulsion. Alcohol, on the other hand extracts the re- 

 sin from their mixtures. Assafoetida, gamboge, and 

 opium, are a few examples of gum resins. 



971. CAOUTCHOUC. GUM ELASTIC. 



Mention the i i ' 



sources and Caoutchouc is a hydrocarbon, obtained from 

 outchouc{ tne milk y J uice f certain trees in Asia, 

 Africa, and South America. This constit- 

 uent of the juice hardens, on exposure to the air, while 

 the remainder is removed by evaporation. By the ad- 

 dition of a little ammonia, the milk may be retained in 

 its liquid condition. Caoutchouc is soluble in ether, 

 spirits of turpentine, oil of coal tar, and many other 

 hydrocarbons. Sulphuret of carbon, a volatile liquid 

 obtained by passing sulphur vapors over ignited char- 

 coal, is also a complete solvent of India-rubber and 

 gutta percha. 



972. VULCANIZED RUBBER. Heated for 



How is caout- 

 chouc vulcani- a snort time with sulphur, at 280, or 



Z ave the^rl - somewnat above this point, caoutchouc 

 ertiesof mil- becomes remarkably changed in its nature, 



canized " rub- -, . ~ , . , , ,, 



ber ? and is no longer stiffened by cold, or soft- 



ened by heat. It is then called vulcanized 



