Page:Experimental researches in chemistry and.djvu/192

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1826.]
177
On pure Caoutchouc.

an essential property of the body, but due to water enclosed within its mass; further exposure to air allows of the gradual dissipation of this water, and then the caoutchouc appears in its pure and dry state, as a perfectly transparent, colourless and elastic body, except it be in thick masses, when a trace of colour is perceived. The change from first to last is best seen by pouring enough of the pure mixture into a Wedge wood or glass basin, to form ultimately a plate of 1/10th or 1/12th of an inch in thickness, and leaving it exposed to air at common temperatures undisturbed.

No appearance of texture can be observed in the pure transparentcaoutchouc; it resembles exactly apiece of clear strong jelly. All the phenomena dependent upon its elasticity, whichare known to belong to common caoutchouc, are well exhibited by it. When very much extended, it assumes a beautiful pearly or fibrous appearance, probably belonging to the effects which Dr. Brewster has observed elastic bodies to produce, when in a state of tension, upon light. When it has been extended and doubled several times, until further extension in the same direction is difficult, it is found to possess very great strength.

Its specific gravity is 0.925, and no reduplication and pressure of it in a Bramah's press was found permanently to alter it. It is evidently pervious to water in a slight degree, or otherwise the interior of a piece of caoutchouc coagulated from the sap would always remain opake. It is equally evident that water passes but very slowly through it, from the time it takes to evaporate that which lies in the middle of a thin cake. It is a non-conductor of electricity.

The pure caoutchouc has a very adhesive surface, which it retains after many months' exposure to air. Its fresh cut surfaces pressed together also adhere with a force equal to that of any other part of the piece.

A strip of it boiled in solution of potash, so strong as to be solid when cold, was not at all affected by it, except that its surface assumed a pearly or tendinous appearance; no swelling or softening, above what would have been produced by water, occurred.

The combustibility of caoutchouc is very well known. When the pure substance is heated in a tube, it is resolved into substances more or less volatile, with the deposition of only a small trace of charcoal; at a higher temperature it is resolved into