The trees and plants whose latices furnish caoutchouc in considerable quantity chiefly belong to the natural orders Euphorbiaceae, Urticaceae, Apocynaceae, Asclepiadaceae.
On standing, some latices separate, more or less readily, into an upper layer resembling cream and consisting of the globules, and a lower watery layer.
This separation can be rapidly effected with some latices by the use of a centrifugal machine, but this method has not yet been applied to any extent commercially.
When exposed to air the latex gradually undergoes putrefactive changes accompanied by coagulation of the caoutchouc. The addition of a small quantity of ammonia or of formalin to some latices usually has the effect of preserving them for a considerable time.
A complete examination of a series of different latices has shown that, in many cases, e.g.
In this respect it differs from gutta-percha, which, like caoutchouc, is derived from the latices of certain plants.
' Cedere squamigeris latices nitentibus aiunt Et liquidas aperire vias, quia post loca pisces Linquant, quo possint cedentes confluere undae.'
Of considerable importance to the value of the rubber is the absence of the resinous constituents which are present in greater or smaller proportion in all latices.
The proportion and nature of the proteids or albuminous materials varies considerably in different latices.