40 CAOUTCHOUCS OR INDIARUBBERS. 
appears as large very uniform slabs, at present valued at 
about 4s. per lb. 
Pure caoutchouc, freshly prepared, is almost colourless, 
but rubber as it appears on the market is of all kinds of 
colours, blackish, blueish, greyish, yellowish. A good rubber 
should be uniform in texture and colour. 
Caoutchouc appears to be a compound of carbon and 
hydrogen, expressed by the formula 0 H x e )n. It is slightly 
lighter than water, having a specific gravity of about 0*92. 
It is a non-conductor of heat and electricity, and becomes 
electrical on rubbing. It is insoluble in water and alcohol, 
but absorbs them, and swells up in so doing. In oil of 
turpentine, carbon bisulphide, ether, benzine, chloroform, 
&c„ rubber forms a clear homogeneous sticky fluid, usually 
known as rubber solution, but which is rather a solution of 
the so-called solvent in the rubber than of the rubber in the 
solvent. 
Rubber alters in air and is injured by oils. When heated, 
*«#•» by standing in the sun while drying, it melts to a sticky 
mass, which does not become firm again on cooling, and 
which is almost valueless. Crude rubbers contain more or 
less resin, which lowers their value; the smallest proportions, 
about 1^—7 per cent., are found in the best rubbers, e.g., in 
Para. Rubber is used in innumerable ways in the arts. The 
crude product goes through various preliminary processes. 
It is first boiled in water for twelve or more hours to soften it, 
then torn to fragments in a machine on the principle of the 
coffee-pulper, and then passed in small quantities between 
grooved rollers moving in opposite directions, which are 
continually washed by a stream of water. The rubber 
emerges as thin sheets with a peculiarly pitted surface, and 
cleaned of its coarser impurities such as bark, sand, &c. 
is then dried and masticated between heated grooved rollers, 
pressed together into sheets or blocks and left in a cool place 
for some months to become homogeneous, and is then cut 
into sheets for manufacturing purposes. 
