This latter figure relates to rubber vulcanised with sulphur, but without other ingredients. So far as mineral ingredients are concerned these are of three types. Firstly, coarse powders such as barytes and chalk, which do little more than dilute the rubber. The tensile properties of such mixtures are similar to those of the unmineralised mixtures, but inferior in proportion as the percentage of rubber is reduced. A second group forms an intermediate class and includes substances such as refined china clay. The product is cheapened, but not deteriorated to the extent produced by constituents of Class (I). The china clay particles are mainly small, and these produce what is known as a reinforcing effect. The rubber becomes stiffer and harder without losing its character. It stretches less, but exerts a greater resistance to deformation. For many purposes it is improved. Class (3) comprises the reinforcing pigments par excellence, in particular " carbon black " or gas black. Some reinforcing pigments such as zinc oxide and carbonate of magnesia produce characteristic effects, as,for instance, in the case of tear. Most vulcanised rubber tears easily if once a nick is made and a tear is started but by using the above-mentioned ingredients this tendency is reduced. There are also numerous organic compounding ingredients, such as glue, bitumens, fatty oils, and sulphurised oils (the so-called rubber substitutes). All these have their specific effects on the quality and properties of the resultant vulcanised rubber. This enables the manufacturer to vary the properties of his product to a surprising extent, and it is this point we wish particularly to bring home to the engineer. Compounding ingredients are not to be confused with adulterants ; in some cases they are used to cheapen the goods, particularly small articles for domestic use, where prices are very cut, but for engineers' require ments where quality is the first consideration this cannot, or, at any rate, should not be done. Compounding ingredients enable a graduation of qualities, from a soft, highly distensible product, e.g., shock absorber cord to a hard, tough, but highly elastic product resistant to deformation such as might be used for a railway buffer or solid tyre. Then, again, with other compounding ingredients it is possible to impart to the rubber a leathery quality for such purposes as pump valves and shoe soles. Perhaps the most characteristic property of rubber is its resistance to abrasion, and for this purpose it has been found far most lasting than steel, as in the lining of ball mills and the floors of dredgers. A sand blast which will remove 1/16th inch of mild steel will not have any apparent, effect on a suitably compounded and vulcanised rubber. In some cases rubber is used more as a binder, as in asbestos packing, five per cent, of rubber sufficing to hold the asbestos and other ingredients together. There is hardly any substance which cannot be incorporated with rubber, and every substance has its specific effect on the vulcanised product. Now that vulcanisation can be accomplished at low temperatures by means of accelerators or in the cold with non-corrosive gases (Peachey Process), vegetable and animal substances can be incorporated with rubber and subsequently vulcanised without damaging them. This has enormously widened the possible field. To-day, for instance, we have brushes with the fibres or bristles set in vulcanised rubber, with the result that the newer solvents used in paints do not loosen the hairs owing to the resistant nature of the vulcanised rubber. Or again, substances such as wood pulp, ground cork, leather waste, etc., can be utilised. Details of these will be given in the next section, but the point to bear in mind is the possibility of displacing almost any material by rubber suitably com- pounded and vulcanised.

In order to illustrate with greater precision the modifications pro- duced by different compounding ingredients we have chosen a few Of the commoner mineral ingredients, and give the load-stretch curves obtained with the vulcanised products. ,

Specimen marked A (see diagram, Fig. 179), is a rubber vulcanised with 10 per cent, of sulphur and without other ingredients, the other graphs correspond to specimens containing mineral constituents and will be discussed later.

It will be noted that the curve is broadly characterised by (1) a first flat portion in which elongation increases rapidly in contrast with the load ; (2) a middle portion of considerable curvature where the load begins to increase more rapidly and overtakes the elongation, and (3) a final flat portion in which the load increases more rapidly relatively to the elongation. .Hence the behaviour of rubber is the reverse of that of metals, which at first show great resistance to stretching, and then lengthen more rapidly as the rupture point is approached.