on the Naturalization of Fishes. 19 



In know not, if I formerly mentioned that the Pleuronectes 

 Zimanda was found in the Loire ; if not, it will be one to add to 

 the lists formerly given. 



ART. III. On Pure Caoutchouc, and the Substances by 

 which it is accompanied in the State of Sap, or Juice., By 

 M. Faraday, F.R.S., Cor. Mem. Inst. of France. 



I HAVE had an opportunity latterly, through the kindness of Mr 

 Thomas Hancock, of examining the chemical properties of caout- 

 chouc in its pure form, as well as of ascertaining the nature and 

 proportions of the other suhstances with which it is mixed, when 

 it exudes as sap, or juice, from the tree. At present much im- 

 portance attaches to this substance, in consequence of its many 

 peculiar and excellent qualities, and its increasing applications to 

 useful purposes. I have thought, therefore, that a correct account 

 of its chemical nature would possess some interest. 



The extensive uses, both domestic and scientific, to which Mr. 

 Hancock has applied common caoutchouc, in consequence of his 

 peculiar mode of liquefying it, are well known. Hence he. was 

 fully alive to the importance of its applications, when in its ori- 

 , ginal state of division. When he gave me th*e substance, he 

 communicated many of his observations upon it, which, with 

 others of my own, form the present paper. 

 The fluid, I understood, had been obtained 



and was very nearly in the state in which it came from 

 the tree ; it had been altered simply by the formation of a slight 

 film of solid caoutchouc on the surface of the cork which closed 

 the bottle. The caoutchouc thus removed was not a 5OOth part 

 of the whole. The fluid was a pale-yellow, thick, creamy- 

 looking substance, of uniform consistency. It had a disagreeable 

 acescent odour, something resembling that 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 ; 



C 2 



