RUBBER AND RUBBER PLANTING 211 



Properties of Colloids. 



A colloid may be defined as a substance which forms 

 a jelly-like solution, incapable of passing through a 

 membrane of parchment or similar material. Colloids 

 are thus contrasted with crystalloids which are able 

 in solution to pass through colloidal membranes. A 

 colloid does not crystallize, and has no definite melting 

 point ; in fact, the physical changes induced in it by 

 heat or by solvents are perfectly continuous so long 

 as the chemical molecule of the substance remains 

 intact. The isolation of a particular colloid from a 

 mixture containing other colloids offers great technical 

 difficulties, and such a substance is exceedingly difficult 

 to obtain in a chemically pure condition. All the 

 ordinary chemical tests of identity are baffled by the 

 consistency of such a material. The colloidal condition 

 is believed to be generally associated with the existence 

 of a very large chemical molecule. 



Composition of technically pure Rubber. 



Technically pure rubber is by no means pure in 

 the chemical sense. The chief impurities present are 

 various resinous and allied bodies, which can be 

 removed more or less completely by prolonged extrac- 

 tion with boiling acetone. The amount of the resins 

 and oily bodies extracted in this way varies from 

 I per cent, upwards, according to the origin of the 



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