MANY OR SPECIAL PURPOSES 355 



could be made to serve the purpose so well if made 

 by any other material. No known substance has 

 the elasticity of rubber, and springs back to its 

 original shape when tension is released. No 

 substitute for the indispensible rubber band is 

 likely to appear. 



The modern rubber industry is built, however, 

 upon the process of vulcanization, which hardens 

 and darkens the crude caoutchouc, reducing its 

 elasticity, in various degrees, according to the 

 proportion of sulphur added, and the thoroughness 

 of their union. Vulcanite is the name given to the 

 product. It is not so sticky as pure rubber, it 

 resists ordinary solvents, and changes of tempera- 

 ture. It is adulterated with pigments and other 

 mineral substances. The colored "rubber goods" 

 in the druggist's window illustrate vulcanite in 

 modern everyday conveniences of the household. 

 Thirty to 70 to per cent, of the weight of rubber 

 tires of vehicles is mineral substance. The rubber 

 plate of false teeth is largely coloring matter. 



Ebonite is the hardest form of vulcanite, black, 

 brittle, shiny. We buy it in combs, photographic 

 trays, and in the insulation of electric apparatus. 



The invention of rubber tires for vehicles has 

 given the rubber market a tremendous lift within 

 recent years. Automobiles consume the greater 



