ELASTIC GUMS 285 



associated mth it arf various substances, regarded as im- 

 purities, among whicli are certain resins. Tliese resins are 

 believed to be derived from the caouteliouc through oxida- 

 tion since they vary eonsideraljl>' in amount and differ chem- 

 ically from the hydrocarbon merely l:iy containing oxj'gen. 

 Such compounds are appropriately called oxidized hydro- 

 carbons, and are distinguished from carboh3-drates by the 

 fact that the oxygen and hydrogen tliej- contain are not in 

 the proportion of H2O. 



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. Tluis a sudden Avarmth is 

 perceptible when a rubber Ijand is f|uick]y stretched in con- 

 tact with the lip. On account of tliis 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-rubljer in being very firm 

 and comparatively inelastic at ordinary temperatures, though 

 at a1)out 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 tvu'pentine, chloroform, naphtha, carbon bisulphid, etc. 

 Unlike caoutchouc, however, gutta-percha is unaffected by 

 fixed oils. 



In chemical composition gutta-jjercha 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 whole 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 ai'e confined to the region of 

 Sumatra and Borneo. Owing to the foolish practice of felling 

 the ti'ees to obtain the milkj^ juice, what was for many years 

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

 tive methods of tapping, similar to those already described 

 for caoutchouc-milk, give a continuous yield for man}'' years. 

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



