Page:Experimental researches in chemistry and.djvu/190

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1826.]
175
On pure Caoutchouc.

of putrescent milk; its specific gravity was 1011.74. When exposed to the air in thin films, it soon dried, losing weight, and leaving caoutchouc of the usual appearance and colour, and very tough and elastic: 202.4 grains of the liquid dried in a Wedgewood basin, at 100° Fahr., became in a few days 94.4 grains, and the solid piece formed being then removed from the capsule, and exposed on all sides to the air until quite dry, became 91 grains: hence 100 parts of sap left nearly 4.5 of solid matter.

Heat caused immediate coagulation of the sap, the caoutchouc separating in the solid form, and leaving an aqueous solution of the other substances existing with it in its first state.

Alcohol poured into the sap in sufficient quantity caused a coagulum and a precipitate, both of which were caoutchouc of considerable purity. The alcohol retained in solution the extraneous matters, which, possessing peculiar properties, will be hereafter described.

Solution of alkali added to the sap evolved a very fetid odour, but did not appear to exert any particular action on the caoutchouc.

The sap, left to itself for several days, gradually separated into two parts; the opake portion contracted upwards, leaving beneath a deep brown, but transparent solution, evidently containing substances very different in their nature from caoutchouc itself, and which, considering the specific gravity of the sap and of pure caoutchouc (the latter being lighter than water), were probably present in considerable quantity.

It was found that, by mixing the sap with water, no other change took place than mere dilution. The mixture was uniform, and had all the properties of a weak or thin sap. Heat, evaporation, acids, and alkali, produced the same effects, generally, as before.

When the diluted sap was suffered to remain at rest, a separation soon took place, similar to that which occurred with the native juice, but to a greater extent; a creamy portion rose to the top, whilst a clear aqueous solution remained beneath. Hence it was found easy to wash the caoutchouc, and remove from it other principles which had been generally involved in it to a greater or smaller extent during its coagulation. For