RUBBER 165 



Vulcanization. — Heat and sulphur produce a profound 

 change in the character of rubber, known as the process of 

 Vulcanization. The ordinary slightly vulcanized rubber 

 corresponds to a formula of about (CioHig)ioS2. but the 

 fully vulcanized rubber, called ebonite, corresponds to about 

 C10H18S2. Mixing is an important part of the preparation 

 of any rubbers for commercial purposes, absolutely pure 

 rubber having little utilit> . Fillers added for some purposes 

 are such materials as pyrites. For increasing mechanical 

 strength zinc oxide, lime, and a few other substances are 

 employed. Asphalte is often used to increase the resistance 

 to water. Pigments of various types are employed to alter 

 the colour. " Oil substitutes " are made by the action of 

 sulphur chloride on oils (see p. 136). Vegetable oils are used 

 for producing low-grade goods. Reclaimed or waste rubber 

 is also much used for admixture. Rubber tubing is generally 

 made either by pressing together the edges of sheet, or by 

 squirting through a die. Canvas and other fabrics built 

 up with rubber constitute an important part of the rubber 

 industry, for all purposes where special strength is re- 

 quired. 



Indigo. — Indigo is grown in Bengal, but is also grown 

 very largely in other parts of India, either for local production 

 of dyestuff, or as a green crop for increasing the amount 

 of organic matter in the soil. It grows very freely, and does 

 not appear to need very much manure, but the problem 

 of the relationship of manure to indigo production has not 

 been by any means completely settled as yet. The plant 

 is cut before flowering, and tied up into bundles. It is 

 carried as quickly as possible to the factories. If it is allowed 

 to ferment, the amount of dye ultimately obtained is reduced. 

 The bundles are filled into a large vat, pressed down by 

 bamboos. The whole is covered with water, steeped for about 

 ten hours, the yellow-coloured liquor tlirown off, and beaten 

 either by hand-working bamboos, or by a kind of paddle 

 wheel. It is then carried to a boiler, where tlie liquor is 

 heated. The indigo Is filtered off, and tlie mass dried. 

 Sometimes a further fermentation is allowed to take place 



