ELASTIC GUMS 285 



associated with it are various substances, regarded as im- 

 purities, among which are certain resins. These resins are 

 beHeved to be derived from the caoutchouc through oxida- 

 tion since they vary considerably in amount and differ chem- 

 ically from the hydrocarl^on merely by containing oxygen. 

 Such compounds are appropriately called oxidized hydro- 

 carbons, and are distinguished from carbohydrates by the 

 fact that the oxygen and hydrogen they contain are not in 

 the proportion of HoO. 



The distinctive characteristic of rubber is its extreme 

 elasticity. A curious result of this is the heat developed 

 when a piece of it is stretched. Thus a sudden Avarmth is 

 perceptible when a rubber band is (juickly stretched in con- 

 tact with the lip. On account of this property means have 

 to be taken in the manufacture of rubber to prevent over- 

 heating when large masses are vigorously worked. 



Gutta-percha differs from india-rubber in being very firm 

 and comparatively inelastic at ordinary temperatures, though 

 at about 50°C. it becomes highly elastic and plastic. It re- 

 sembles caoutchouc in flexibility, toughness, poor conduc- 

 tivity of heat and electricity, imperviousness to moisture, 

 insolubility in dilute acids and in alcohol, and solubility in 

 oil of turpentine, chloroform, naphtha, carbon bisulphid, etc. 

 Unlike caoutchouc, however, gutta-percha is unaffected by 

 fixed oils. 



In chemical composition gutta-percha consists like caout- 

 chouc, of a hydrocarbon similarly associated with resinous 

 substances presumably derived from it by oxidation. Unless 

 well purified soon after being collected the change into resin 

 may go so far as to make the Avhole mass worthless. 



Gutta-percha is obtained from several different species 

 of trees all closely related to the taban-tree (Fig. 272) which 

 was the original source. All are confined to the region of 

 Sumatra and Borneo. Owing to the foolish practice of felling 

 the trees to obtain the milky juice, what was for many years 

 the main source of supply is now destroyed. More conserva- 

 tive methods of tapping, similar to those already described 

 for caoutchouc-milk, give a continuous yield for many years. 

 It has been also found that gutta-percha of the finest quality 



