The Romance of Modern Chemistry

MORE ABOUT FUEL portion of the heat reappears in the form of useful work. In this respect the internal combustion engine, such as is used on a motor car, is much superior to the steam engine, by which we convert only about 10 per cent, of the heat value of the coal into power. The power of different fuels to give out heat when burned—the calorific power, as it is called—varies very con- siderably. The heat given out in the combustion of one pound of coal, for example, is nearly twice as great as that liberated when one pound of dried wood is burned. The calorific power of petroleum, on the other hand, is nearly 30 per cent, greater than that of coal. In selecting a fuel, however, many other factors have to be borne in mind besides the calorific power; the prudent engineer has to consider the bulk of the fuel, its cost, its handling, and the readiness with which it may be fed into the engine. It is the total effect of all these factors on the balance-sheet that is the important thing from the commercial point of view. From what source has all the energy latent in naturally occurring fuels been derived ? George Stephenson, when he was asked what drove his locomotive, replied that it was “bottled-up sunshine,11 and he was not far wrong. The reader will ask how the bottling process was carried out; but that is another story, which must be postponed to a later chapter. 153