A vast improvement in the properties of vulcanised rubber is obtainable by the use of suitable accelerators, and higher breaking loads are obtainable. Thus, with carbon black breaking loads up to nearly 4,000 lb. per sq. inch are obtainable, the rubber stretching to five and a half times its original length under this load at the moment of rupture, or about 10 tons calculated on the cross-sectional area at the moment of rupture.

Rubber differs from other materials, not only in its high degree of distensibility, but in regard to its elastic limit, whereas metals, wood, etc., reach their elastic limit some time before rupture, the elastic limit of rubber is coincident with its breaking point.

The work done on stretching, the work given out on retraction and hence the energy storage capacity of rubber is extraordinarily high, and explains the advantage shown in practice by rubber over steel springs, as, for instance, in a rubber buffer or a rubber tyre over a spring wheel. Geer, in his book The Reign of Rubber, has put this concisely. He says : " If, in a machine, one pound of tempered spring steel be stretched just to the elastic limit-an action which would require a bar an inch square in section and weighing one pound to be loaded with 82,000 pounds-one can store in it 95-3 foot-pounds of energy. H ickory wood, when pulled along the grain, is elastic enough to permit the storing of 122-5 fool-pounds at its elastic limit. In this way, our pure gum rubber-compounded without the accelerator that we have already described, would permit us to store in it a matter of 3,186 footpounds." By modern methods, using suitable accelerators, we can, he says, store 7,633 foot-pounds, and by incorporating suitable " reinforcing pigments" we may obtain still higher figures-e.g., 7,988 footpounds, using zinc oxide and 14,887 foot-pounds with carbon black.

To the popular mind, little distinction probably exists between one rubber factory and another, and it is even doubtful whether the sharp line of demarcation is recognised which exists between the methods employed in the production of raw rubber and those used for its conversion into articles for everyday use.

The manufacture of rubber can scarcely be regarded as a simple industry, and is better pictured as a group of closely related trades which employ the same principal raw material and subject it to the same preparatory and final operations, differing in regard to the type of article produced and the intermediate processes used in its manufacture.

As has already been pointed out, rubber must first be rendered plastic before it can be shaped into serviceable articles : while, in addition to adding the sulphur necessary for vulcanisation, it is necessary for many purposes to incorporate various ingredients to modify the colour and other physical properties.

The general methods and machinery used for what is termed the milling and compounding of rubber are much the same in every section of the industry, though naturally, differences exist in regard to the constituents which are admixed with the rubber. Once the compounding is completed and the rubber passes to the making-up stage, the differences between the various branches of the industry become apparent.

Take for example, the methods used for the manufacture of moulded ebonite and rubber shoes. At every stage the machinery and processes are entirely different, the methods of vulcanisation bear little resemblance to each other, and the physical characteristics of the final products have little or nothing in common. This possibly may be an extreme case, but similar conditions obtain to a lesser degree in most of the other sections into which the industry is divided, and a long apprenticeship is necessary before anyone can justly claim to possess an all-round knowledge of even a few of the principal branches.

This complexity which characterises the rubber industry will be apparent from the rough classification of its activities :-

1. Balls and toys. 8. Belting.

3. ;Boots, shoes and heels.

4. ;Cut sheet and rubber thread.

5. ;Ebonite.

6. ;Electric cables and wires.

7. ;Hose.

8. ;Made-up articles such as hot-water bottles and football bladders.

9. ;Moulded or Mechanical goods.

10. ;Packings.

11. ;Rubber covered rollers.

12. ;Rubber flooring, tiling and mats.

13. ;Solid and pneumatic tyres and tubes.

14. ;Sponge rubber.

15. ;Surgical and dipped goods.

16. ;Waterproof fabric and garments.

Each of these sections has its own particular problems and difficulties ; each has its own special machinery and methods, and each has to produce a wide range of products to satisfy the innumerable applications of its particular market.

This rough outline of the rubber industry as it exists to-day may, perhaps, convey some idea of its complexity and the enormous variety of its products.

A hundred years ago rubber had but one application involving the use of a few tons of rubber each year. To-day the world's consumption of raw rubber is in the neighbourhood of half a million tons annually, and its uses almost beyond computation.

Prom what has already been said it will be clear that to endeavour to enumerate in a few pages the many applications of rubber in the engineering and allied industries would be an impossible task. It would be equally difficult to attempt to give a list of all the industries in which rubber is used, and we must therefore content ourselves with a brief discussion of the properties of rubber which are of importance to the engineer.

Before doing so, however, it might be well to digress for a moment to recall the state of techni cal knowledge some hundred years ago, and to contrast the relative positions of the engineering and rubber industries then and now.

A century ago we were on the threshold of the extraordinarily inventive period which was to transform Great Britain from an agricultural country to the foremost centre of engineering and industrial activity.