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 almost 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., unvulcanized) 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 examining 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 electrical 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 vulcanized 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
