806 THE POPULAR SCIENCE MONTHLY. 



African ball, being tolerably firm in consistency, while the African 

 flake, which you see here, and the African tongue, represent the lowest 

 and most viscous qualities of commercial rubber. 



The commercial value of the various qualities of rubber may be 

 estimated, to a certain extent, by noting the loss which the samples 

 undergo during the operation of washing, and also by noticing how 

 far the various samples are softened by a long-continued gentle* heat. 

 Here are some samples which have been heated for some hours in this 

 water-oven ; you will notice that the African tongue has become al- 

 most as soft as treacle, while the Para rubber still retains its form 

 and much of its consistency. 



Caoutchouc is nearly colorless, and when in thin leaves tolerably 

 transparent. It, like very many other substances, contains nothing but 

 carbon and hydrogen, but its properties differ very widely from those 

 of other hydrocarbons almost identical in composition. It has been 

 found to contain, in one hundred parts, 12'5 of hydrogen and 87*5 of 

 carbon. Caoutchouc, as might be supposed, burns very readily and 

 leaves no residue ; if I set fire to a few ounces, you see how it blazes 

 up. It is soft, and very imperfectly elastic, in the true sense of the 

 term that is to say, it does not return to its old dimensions after 

 having been considerably stretched. Here is a strip of pure (i. e., un- 

 vulcanized) caoutchouc a foot long ; you see that I have stretched it 

 to a length of three feet, and, after holding it stretched for a few 

 seconds, I relax it. It now measures, as you see, several inches over 

 the foot. The elasticity of caoutchouc may be enormously increased 

 by vulcanization. 



As regards the stretching of India-rubber, there is a point at which 

 it requires a greatly increased force to stretch it, and at this point it 

 seems to become fibrous in texture, as you may perceive by examiniug 

 this extended sample by the aid of a magnifying-lens. India-rubber 

 has valuable electrical properties, as you are no doubt aware, it being 

 an admirable insulator, and having a great tendency to become elec- 

 trical by friction. 



Freshly cut surfaces of India-rubber cohere very strongly when 

 brought into contact, and this is well illustrated by the old way of 

 making a tube of unvulcanized caoutchouc. You see that I wrap a 

 sheet of caoutchouc round a mandrel, so that the edges project parallel 

 to each other. These parallel edges being cut off by means of scissors, 

 the freshly cut edges adhere, and a perfect tube is the result. Toy 

 balloons are made in a somewhat analogous manner, and are cold vul- 

 canized afterward. 



Either French chalk or soapy water is of constant use in the rubber- 

 factories, to prevent the adhesion of new surfaces of caoutchouc to 

 each other, or to other substances. 



Cold has a remarkable effect on caoutchouc, rendering it rigid and 

 inelastic, and this circumstance considerably detracts from the value 



