When immersed in liquids raw rubber takes up something from most of them. Thus in water it gradually absorbs 10 per cent, or 15 per cent, of its weight, becoming opaque. The water can be dried out again. It takes up less alcohol, but ether is taken up until the mass becomes so swollen and fluid that it mixes with the remainder of the ether or in common parlance the rubber dissolves. This swelling process precedes the solution of rubber whatever solvent be chosen. The ready solubility of raw rubber in various organic solvents, such as low boiling petroleum, benzene, carbon tetrachloride, and other liquids, is of great importance in the technique of rubber manufacture, as by subsequent evaporation ot the solvent thin sheets of rubber can I be prepared and fabrics can be impregnated. Much progress has recently been made by replacing rubber solution by rubber emulsions, in particular that found in nature ready formed, which we call latex.

It has been stated that the properties of raw rubber can be modified by mechanical treatment such as " milling " or " mastication." This is effected by means of milling machines or masticators. The former generally consist of two heavy rollers moving at different speeds or of a roller and a casing. The rubber is continuously passed between the two rollers or the roller and the casing. The effect is gradually to destroy the tensile properties and resiliency of the rubber, which becomes soft and plastic. In this condition it is much more readily soluble, less solvent being required in preparing a sheet or impregnating a fabric. The milled rubber does not recover its original properties, or very imperfectly, except by subsequent vulcanisation.

On account of the sensitiveness of raw rubber to changes of tem-perature, solvents, etc., and the difficulty of manipulation without destruction of its valuable properties, it is not adapted for manufactured rubber goods except in a few instances in which the raw rubber in the state in which it leaves the plantation is directly applicable, as in the case of crepe rubber soles. The only possibility of shaping raw rubber to the required purpose consists in the use of very large, and, therefore, uneconomical, quantities of expensive solvents which are difficult to recover. Alternatively, the operation can be carried out with the liquid latex, which is then allowed to set in the required shape or form. It is possible that this latter process may bulk more largely in the future when the technique of incorporating powders with latex and removal of moisture from the resulting mass has been more thoroughly studied. However this may be, the great bulk of rubber goods are to-day manufactured from the raw rubber and not direct from the latex. The technique of rubber manufacture rests mainly on two phenomena :(1) The profound effect produced by vulcanisation, whereby the rubber, although milled till it has lost its valuable properties, is rejuvenated and yields a product which excels the original raw material in tensile properties. (2) The effect of incorporation of foreign substances, mainly finely divided mineral matter, in the rubber previous to vulcanisation. The incorporation of foreign substances can only be effected by first milling the rubber to make it plastic, and as the sulphur necessary for vulcanisation can be incorporated with the rubber in the same operation, it follows that the first operations in the factory always consist of milling or mastication followed by mixing. The resultant product is then run in sheet form. It is plastic and coherent. From the sheets the article to be manufactured is built up. Alternatively the mixed rubber is spewed, and this being a cheaper operation than sheeting, is adopted as generally as possible.

The process of vulcanisation cannot be described here in detail; suffice it to say that when the mixture of rubber, sulphur, and other ingredients is heated (generally to a temperature of IW° to 150° C. for one half to three hours), the change in properties is brought about. By the use of small quantities of substances known as accelerators the temperature and time of heating can be considerably reduced, and it is possible to vulcanise satisfactorily at tempera -tures of 100° C. and even lower. As a rule, air must be excluded either by enclosing the rubber in a mould or by placing naked in a vessel (vulcaniser) into which steam at sufficient pressure is passed to maintain the desired temperature (open cure). By suitable compounding-i.e., inclusion of certain ingredients such as litharge-rubber may also be vulcanised naked in ovens open to the air. Vulcanising with sulphur is generally described as heat vulcanisation or " hot cure " to distinguish it from another method in which the sulphur is omitted and the rubber is exposed to the action of sulphur chloride, either as a vapour or in dilute solution in carbon bisulphide. This method, known as the " cold cure," is only suitable for thin sheets, as the action is confined to the surface layers. The same considerations apply, although to a less extent, to the Peachey Process, in which vulcanisation is brought about by consecutive treatment with hydrogen sulphide and sulphur dioxide gases. Finally mention should be made of the Schid-rowitz process, whereby the rubber globules or particles in the latex are vulcanised. On drying down or coagulating the rubber is obtained in the vulcanised state.

Vulcanised rubber differs mainly from raw rubber because of its enhanced physical properties, greater indifference to change of temperature and insolubility in all solvents for raw rubber. The incorporation of mineral ingredients (so-called pigments) and other materials has a profound effect on the physical properties as in the case of compounded raw rubber. Whereas the breaking load of raw rubber may reach 40 kilos, per sq. cm. that of vulcanised rubber may exceed 200 kilos., and the elongation at break, five and ten respectively. If the breaking load be calculated on the cross sectional area at the moment of rupture we have breaking loads of 200 and 2,000 kilos, per sq. cm. or 2-6 and 26 tons to the sq. in. respectively-figures which compare not unfavourably with metals.