2 ( JS CUKIOUS OR USEFUL PLANTS. 



carbonic acid gas, hydrogen gas, an empyreumatic oil, and ammo, 

 nia ; the residue is a trifling quantity of coaly matter. It may be 

 dissolved by heat in fat oils, and will combine also with heated 

 wax. Dr. Roxburgh dissolved it in cajeput oil. Exposure to the 

 air does not affect this substance; and it is not soluble in either wa- 

 ter, or alcohol : but if it be boiled in water, the extraneous matter 

 near its surface is carried off, and two or more pieces of it may then 

 be joined together by mere pressure. It is completely soluble in 

 ether, if the ether be previously washed in water ; and as the caout- 

 chouc remains unaltered on the evaporation of the ether, this solu- 

 tion may be employed to form it into any shape desired, though it is 

 too expensive for common use. A more simple method of forming 

 tubes of it is to split a piece of cane, and between the pieces intro- 

 duce a slip of whalebone : if the caoutchouc be then cut into slips 

 and twisted closely round the cane, and heat be applied, the whole 

 will unite into one piece or tube, from which the whalebone and the 

 cane may easily be separated. Some wind a thread round the 

 caoutchouc, when twisted round the cane for the purpose of uniting 

 the joinings, and apply the heat of boiling water. Caoutchouc is 

 also soluble in rectified petroleum ; which, in like manner with 

 ether, leaves it unaltered on evaporation. It has been thought to 

 be insoluble in alkalies; but Dr. Thomson proved the contrary by 

 accidentally dissolving it in ammonaical gas. Several of the acids 

 act upon it, as the sulphuric and the nitric ; but others do not, as 

 the muriatic. The specific gravity of caoutchouc is 0*9335. 



The very ingenious Mr. John Gough of Middleshaw, near Ken- 

 dal, has made some interesting experiments upon caoutchouc, from 

 which it appears that if a thong of this substance be put into warm 

 water till it becomes quite pliant, and then applied to the lips to 

 estimate its temperature ; and if it be then removed, stretched for- 

 cibly, and brought again to the lips, while extended; a very sensible 

 increase of temperature will be perceived, which, however, will 

 immediately sink on allowing it to return to its former state. It 

 appears also that if the thong be plunged into cold water immedi- 

 ately upon being stretched and held there, it will be found to have 

 lost much of its contractile power, for it will not on letting go one 

 end return to its former dimensions ; but if it be placed in warm 

 water, or held some time in the warm hand, it will re-acquire its 



