ELASTIC GUMS 285 
associated with it are various substances, regarded as im- 
purities, among which are certain resins. These resins are 
believed to be derived from the caoutchouc through oxida- 
tion since they vary considerably in amount and differ chem- 
ically from the hydrocarbon 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 H,O. 
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 warmth is 
perceptible when a rubber band is quickly 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, ete. 
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 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 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 
