Its specific gravity is 1.012.
Caoutchouc itself or raw india rubber is a mixture of several hydrocarbons of the following composition in general:
| Carbon | 87.5 |
| Hydrogen | 12.5 |
| 100.0 |
Its specific gravity is from .912 to .942.
The hydrocarbons composing it are isomeric or polymeric with turpentine. This fact brings it well within the range of familiar vegetable products. As will be seen the products of its distillation fall among the same polymers and isomers.
When pure it is nearly colorless, the dark color being due to impurities. In thin sheets it is almost or quite transparent. It burns readily, and with a very luminous, smoky flame, as might have been anticipated from its composition. The action of heat and cold on it is dependent on the degree of the temperature. At ordinary temperature it is elastic and firm. It can be stretched and will return almost to its original size when released from tension. Yet the return to its shape is so liable to be incomplete, especially after long sustained stretching, that pure unvulcanized india rubber is considered imperfectly elastic.
Any elasticity it possesses is principally elasticity of shape as distinguished from elasticity of volume. In other words when pressed or stretched it may change shape to a great extent but hardly change its volume at all. A cube of 2½ inches under a weight of 200 tons lost 1-10 of its volume only. This is largely due to the fact that it represents an approximately solid body, or one destitute of considerable physical pores. Solids and liquids are very slightly compressible. Whatever degree of compressibility caoutchouc possesses is due principally to its minute pores.
If the temperature is reduced to the freezing point of water a piece of raw india rubber becomes rigid and stiff. On application of heat it returns to its former pliable condition. The same return to flexibility may be brought about by stretching it mechanically. This may be rather a fallacy. Stretching india rubber warms it, so that in this mechanically imparted rise of temperature we may find at least a probable cause of the softening.
If the temperature is raised several effects are produced, according to circumstances. A piece which has been stretched and held stretched, has its tension increased by a degree of heat considerably less than that of boiling water. Some offer the theory that it contains air enclosed in its pores which, expanding, produces this effect. As the boiling point is reached the material softens and becomes somewhat plastic, so that it can be moulded into shape to a considerable extent and stretched to threads of great fineness. Its elasticity also disappears as the heat is maintained. These effects increase in extent up to a heat of 248° F. (120° C.). The return to its original state is not immediate however. Some time is required before the reduction of temperature will have full effect.
If now a still higher degree of heat is applied, 392° F. (200° C.) the india rubber softens to a viscous body, or melts. From this state it cannot be restored. It remains permanently “burned” or melted whatever is done to it. Some attempt at hardening may be made by the use of vulcanizing chemicals, but the result will be very imperfect.