1826.] On pure Caoutchouc. 177 



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 arid 

 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 Wedgewood or 

 glass basin, to form ultimately a plate of y^th or -^ th of an inch 

 in thickness, and leaving it exposed to air at common tempe- 

 ratures undisturbed. 



No appearance of texture can be observed in the pure trans- 

 parent caoutchouc ; it resembles exactly a piece of clear strong 

 jelly. All the phenomena dependent upon its elasticity, which 

 are 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 di- 

 rection 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 sur- 

 faces 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 as- 

 sumed a pearly or tendinous appearance ; no swelling or soften- 

 ing, 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 sub- 

 stances more or less volatile, with the deposition of only a small 

 trace of charcoal ; at a higher temperature it is resolved into 



N 



