Page:Experimental researches in chemistry and.djvu/191

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176
[1826.
On pure Caoutchouc.'

this purpose a portion of the sap was mixed with about four volumes of water, and the mixture put into a funnel, stopped below by a cork; in the course of eighteen or twenty-four hours, when the caoutchouc had risen to the top and occupied about its original volume, the aperture at the bottom of the funnel was opened and the solution drawn off; more water was then added to, and mixed with, the caoutchouc, and the operation repeated, and this was done four or five times, until the water came away nearly pure. During the latter washings, the caoutchouc required a longer time to rise to the surface, in consequence of the decreasing specific gravity of the solution in which it was suspended. This was obviated at times, according to the experiments for which the caoutchouc was required, by performing the first washings with solutions of common salt, muriatic acid, &c., and ultimately finishing with pure water.

In this way the caoutchouc was purified without any alteration of its original state. It now appeared in its state of mixture with water perfectly white: portions of it left for a twelvemonth over water underwent no change in that time, except coagulation and a slight film upon the surface; the rest was as miscible with the water as at first, and when coagulated, equally elastic. The sap or the washed caoutchouc is much more easily preserved in the diluted than in the concentrated state.

It produced no particular appearance with the solutions of iron or other metals.

When evaporated, either on paper, in a capsule, or otherwise, the caoutchouc was left in its elastic state, and perfectly unaltered, except with respect to purity. When put on to absorbent surfaces, as bibulous paper, chalk, or plaster of Paris, the water was rapidly abstracted, and the caoutchouc almost immediately united into a mass, retaining the form of the thing on which it was cast. In this way Mr. Hancock has made beautiful medallions with the sap. Poured on to a filter, the water passes through, and the caoutchouc coagulates.

When aggregated in any of these ways, the caoutchouc appears at first as a soft white solid, almost like curd, which by pressure exudes much water, contracts, becomes more compact, has acquired elasticity, but is still soft, white and opake. It also attains this state without pressure, if time be permitted for the water to evaporate. The opacity belonging to it is not