Rubber is a vegetable product obtained from certain tropical trees, mainly the herea braziliensis or the para rubber tree, by making incisions in the " bark " or cortex. This part of the rubber tree is provided with a system of tubes or vessels which are not present in other sorts of trees. When cut these vessels exude a milky fluid, or latex, which consists of an emulsion of rubber globules, just as milk is an emulsion of butter fat globules. On stirring a little weak acid such as acetic acid into the latex, and setting aside, the whole sets like a junket, which gradually contracts, leaving a clear whey or serum. This is known as coagulation. The junket-like mass is put through roller machines to express as much as possible of the serum, which contains substances of little value. Two modifications of the process are usually employed. In one the rubber is coagulated to give rectangular cakes which, after passing through light, even-speed rollers, are hung in smoke till dry. This yields the smoked sheet of commerce. In the second process the rubber is put through a series of powerful differentially geared roller machines under a stream of water which washes away the expressed serum. Some of the machines are grooved to give a better grip on the soft coagulated mass. The final product consists of thin sheets with a grained or corrugated surface. These are hung in air till dry and yield the pale crepe rubber of commerce. The freshly coagulated latex is white and opaque, but on drying becomes translucent. There are other methods of preparation, some old and little used, others recently introduced, but they need not be considered here, as they yield but a small proportion of the raw rubber on the market. It should, however, be mentioned that in the preparation of rubber a certain amount of latex coagulates spontaneously at various stages of collection or becomes contaminated with bark, soil, or other extraneous dirt. Such residues, termed scrap, are washed and worked up to give a crepe rubber more or less dark or brown in colour. There is also available a large amount of brown crepe rubber, the so-called amber blanket, prepared by washing native coagulated latex.

It will be noted that the difference between sheet and pale crepe rubber is not great. The sheets are smoke-dried and contain more of the serum than the pale crepe. Dark crepe rubber is not so clean and of less reliable quality than the pale, but for some purposes is preferred.

Until the introduction of the crepe rubber sole, rubber was hardly used in the raw state, because better products were obtained by subjecting it to the process of vulcanisation. This we deal with in the next section. It must not be forgotten that plantation crepe and sheet when placed on the market were new materials. Crepe was made, it is true, twenty years ago, in preplantation times, by the rubber manufacturers in the course of washing and cleaning the so-called " wild rubber," a moist and frequently very dirty native product which formed the whole raw material in those days. But the treatment was drastic, and the raw material mostly inferior, so that the crepe rubber had not the physical properties of plantation crepe, and was, therefore, less suited for use in an unvulcanised condition. This arises from the peculiar effect produced on raw rubber by mechanical " working," or the process commonly referred to as milling or mastication, a preliminary in the vulcanisation process. Raw rubber is " elastic " in the non-technical sense of ' stretchable." In this respect it is inferior to elastic thread or a high-grade rubber band-nevertheless, it will stretch to four or five times its original length before rupturing. Its tensile strength may be anything up to three tons per square inch, but as neither sheet nor crepe are of uniform structure-that is, the sheet contains numerous pores, and the crepe is the result of tearing apart in little pieces, which are pressed together again-it is obvious that the above figure is probably appreciably lower than would be given by a really homogeneous material. When " pure " rubber is stretched its volume remains substantially unaltered, so that when stretched to, say, five times its original length, the cross section is one-fifth of that of the unstretched material. Hence, if we calculate the breaking load on the sectional area at the moment of rupture the tensile strength must be put at five times the above figure, namely, fifteen tons per square inch. It is, however, in its resistance to abrasion that raw rubber shows to the greatest advantage. Apart from the use of raw crepe and sheet for shoe soles, where it has proved superior to leather from the point of view of durability, raw rubber has also been used for the floors of dredgers, where its life exceeds that of steel many times, for pump valves, and similar articles, etc. For these purposes it has been found advantageous to incorporate mineral powders with the latex, and the product can then be worked up into smooth sheets externally resembling linoleum, but of an exceedingly tough, leathery consistency. The " Wilkinson " process is a recent development which has produced a type of rubber which has met with success in these directions. It should be emphasised that the physical properties of these products are of a different order from the bulk of manufactured-i.e., vulcanised-rubber goods, and it is essential that the right type of rubber be chosen for each requirement. Raw rubber, whether compounded with mineral matter (added in the latex state), or " pure " possesses certain properties which make it unsuitable for many uses. These are largely or entirely obviated by vulcanising the rubber, as described in the next section. They comprise mainly the sensitiveness of the material to the effects of temperature and solvents. When cooled, raw rubber tends to harden and its specific gravity increases. The change is hardly noticeable, because it is so slow, until the temperature falls to approximately 5° C. Kept at this temperature for some time the specific gravity recovers to normal, but the rubber becomes opaque and hard. At 0° G. the change is rapid-a few hours suffice to convert the relatively soft translucent and distensible crepe or sheet into a hard, opaque and barely flexible mass. On warming to 30°-35° C. the rubber " thaws " and reverts to its original soft translucent state. At higher temperatures the change is more rapid. The " thawing " may also be produced by mechanical working, such as by repeated bending or rubbing of the sheet. As, however, rubber is a remarkable non-conductor of heat, it becomes a tedious process to thaw a case of massed rubber which has become " frozen " during transit to Europe in the winter. When the temperat ure is unduly raised- sav to 100° C.-raw rubber be-comes soft and sticky, nor does it recover completely on cooling. At higher temperatures more or less complete decomposition with the production of sticky semifluid masses results. Hence, raw rubber is only suited for use within a relatively narrow temperature range.