DISCOVERY 



187 



made in the bark of the growing tree, and is collected 

 in small tin cups which are cemented to the trunk 

 with clay. On emerging the juice is whitish, but 

 darkens and coagulates, with time and exposure, into 

 a dingy brown mass. The shapes in which it is 

 imported vary largely according to circumstances; 

 some being in the form of round balls, known as nigger 

 heads, and other in blocks, thimbles, cakes, and bottle- 

 shaped masses. Chemically, rubber or caoutchouc is 

 a complex substance and consists of several different 

 gums. These bodies have varying degrees of solubility. 

 Water, of course, has no action upon raw rubber, 

 but it dissolves in a mi.xture of carbon bisulphide, 

 benzene, and naphtha. Xo single solvent will dissolve 

 it entirely, some of its constituents being acted upon 

 more quickly than others by the various solvents 

 used, and the raw substance seems to consist of a 

 highly porous network of cells. At a temperature 

 of 10° C. raw rubber is a sohd body with very little 

 elasticitj', whilst at 36° it is soft and elastic to a high 

 degree, and is capable of being stretched to sLxteen 

 times its length. Further increase of temperatures 

 lessens these elastic qualities, and at 120° C. it melts. 

 In the raw state rubber has several peculiar properties, 

 one of which is that, after being stretched and cooled 

 suddenly in the stretched condition, it retains its new 

 form, and only regains its former shape on being warmed. 

 .\nother striking feature is its strong adhesive capacity ; 

 this is so powerful that the raw substance cannot be 

 cut with a knife unless the blade is wet, and freshly cut 

 i portions, if pressed together, form a homogeneous mass. 

 The first use of rubber gum was for waterproofing 

 1 i; lies, and in 1S23 a solvent was found by Charles 

 Mackintosh which allowed a thin coating to be applied 

 like paint. Owing, however, to its sticky nature, it 

 Imade very little headway for waterproofing purposes, 

 'until, in 1839, an .•\merican named Goodyear found 

 that a mixture of rubber and sulphur heated together 

 produced a body very different from the raw gum, 

 being elastic at low temperatures and free from sticki- 

 mss right up to its melting-point. This discovery, 

 known now as vulcanising, opened up an important 

 [future for rubber, and by varying the amount of sulphur, 

 [together with the heat and duration of the process, 

 'products are obtained which range from the softest 

 flexible tubing up to the material used for making the 

 jarrels of fountain-pens and the mouthpieces of pipes. 

 Dn broad lines the outline of the vulcanising process 

 s as follows : The raw material is well masticated 

 between hot rollers, and is incorporated with the 

 lecessary amount of sulphur ; it is then pressed into 

 noulds which clamp together and prevent any loss 

 if shape owing to the contraction, of the rubber. 

 leat is now applied, and, when the process is over, 

 he rubber will be found to have taken the shape of the 



[Continued on A<S3 



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