OR INDIA RUBBER. 55 



chouc, producing carbonisation and imparting a peculiar softness to the 

 articles so treated ; aided by beat, it is rapidly decomposed with the 

 disengagement of sulphurous acid, and complete destruction and carbonisa- 

 tion of the caoutchouc. Nitric acid of ordinary strength, contrary to 

 general statements, exerts an action upon caoutchouc, and when aided by 

 slight increase of temperature causes violent decomposition. Nitric oxide 

 and nitrogen are evolved, while carbonic and oxalic acids are formed, 

 the latter of which may be crystallised out of the solution. Chromic acid 

 aided by heat also exerts a powerful action upon caoutchouc, reducing it 

 to a soft and gelatinous substance. Concentrated solutions of the alkaline 

 chlorides have been found to exert no action upon caoutchouc or any of 

 its known compounds, even after immersion for several years. Metallic 

 chlorides possess the property of acting in some peculiar manner upon 

 its surface, imparting a softness and immunity from the otherwise rapid 

 action of solvents and oils. Caustic alkalies exercise no action upon 

 caoutchouc, even after long periods of immersion. Alkaline sulphides, 

 under heat alone, produce an effect analogous to vulcanisation. 



Caoutchouc is soluble in most of the fluid hydrocarbons ; coal-oil and 

 its products, benzole and naphtha, ranking as the best : next in order we 

 have bisulphide of carbon and perchloride of formyle (chloroform), and 

 lastly, oil of turpentine. Oleaginous and fatty substances all more or less 

 act upon caoutchouc, producing, in contradistinction to the first-named 

 solvents, varnishes of a non-drying character. 



Immersed in distilled water, caoutchouc undergoes no alteration beyond 

 slightly increasing in weight, from 5 to 12 per cent. ; but after a few days' 

 immersion, the parts most exposed to the water change their colour to 

 a white or bleached hue : the stratum so affected is specifically heavier 

 than the interior of the mass. This effect is caused by a process of sur-' 

 face hydration, arid not by the direct absorption of water; for under no 

 condition, not even token aided by long immersion and extreme pressure, 

 does water penetrate beyond a fractional distance into the substance of the 

 caoutchouc. 



Immersed in water holding alkaline salts in solution, this hydration 

 does not manifest itself in an equal degree, and has not been found to 

 exceed more than 3 per cent. 



It is quite a matter of question, whether this process is not due 

 entirely to " endosmose," in place of the generally-received doctrine of 

 absorption. Leopold Gmelin^ in his ' Handbook of Chemistry,' vol. i., 

 p. 28, (published by the Cavendish Society in 1848,) says, " Through 

 any material covered with caoutchouc, endosmose proceeds from alcohol 

 to water, first slowly, but afterwards quickly, when the caoutchouc has 

 been acted upon by the alcohol : at the same time the alcohol becomes 

 more and more dilute by the action of an opposite stream of water. A 

 caoutchouc bottle filled with ether gradually empties itself in alcohol or 

 water : if filled with alcohol, it distends in ether, but empties itself in 

 water ; if filled with water, it distends when placed either in alcohol or 



