56 CAOUTCHOUC, OR INDIA RUBBER. 



ether. Hence, caoutchouc is more pervious to ether thau to alcohol, and 

 lastly, water? 



The most marked change in the alteration of caoutchouc is observed 

 when it is suspended in the atmosphere and exposed freely to the direct 

 action of the solar ray : it then undergoes a process of softening and direct 

 oxidation, passing into a viscid mass, with the ultimate loss of elasticity 

 and tenacity. In this state, analysis shows it to consist of — 



C. 66-30 



H. 1125 



O. 22-45 



100-00 

 and also to contain a soft resinous matter in variable quantities, averaging 

 from 11 to 14 per cent., soluble in and extractable by alcohol. This 

 change in its condition does not appear to be caused by any peculiar 

 action of the solar ray, or actinism alone, the same effect taking place 

 when it is similarily exposed to the red, blue, and yellow ray separately 

 and at a temperature of 80° Fahr. Heat or increase of temperature, con- 

 jointly with light and atmospheric influence, in the absence of all moisture, 

 appears to act as the chief exciting cause or agent in setting up this action ; 

 for it is observed that when moisture is present, the tendency to decom- 

 position or oxidation is considerably diminished, and when wholly immersed 

 in water, so as to be perfectly excluded from atmospheric action, it is altogether 

 checked, and never occurs. Varnishes of an unoxidisable nature applied to 

 the surface of caoutchouc also protect and preserve it intact, when thu& 

 exj)osed to the atmosphere. 



We have observed that caoutchouc softens and becomes viscid when 

 exposed to the sun ; but this softening must not be confounded with a 

 process of melting : the former result is due, as has already been stated, to 

 direct oxidation with the formation of resin, while the latter requires an 

 artificial temperature ranging from 240° to 250° Fahr., and in all cases 

 varying with the description of material under trial. At this temperature,. 

 in common with most gums and resinous substances, caoutchouc begins 

 to soften and melt, and upon the application of increased temperature, 

 varying from 300° to 600° Fahr., it is totally decomposed ; its constituent 

 elements become broken up and re-arranged into new groups, having the 

 form of oily fluids, differing considerably both in their density and 

 boiling-points, and upon analysis affording nothing but carbon and 

 hydrogen, differing in proportions only. Some of these oils boil at 90° 

 Fahr., the lightest being highly limpid and colourless, with a specific 

 gravity of 670 ; others have their boiling-points at 600° and 680° Fahr., 

 and approach very closely to the composition of oil of turpentine, 

 C 10 H s : little or nothing, however, is known concerning them. 



Caoutchouc known as " masticated India rubber " is more prone to the 

 direct action of the sun and oxidation than is the native or natural sub- 

 stance ; but in this special feature much is due to the manipulation it has 



