Unfortunately this spirit has less heating value than petrol. That means that a given quantity of each liquid will produce more heat in the case of petrol than in the case of alcohol. Indeed the difference is about two to one. Hence an engine to give out a certain horse-power would need to have its cylinders twice as big if it were to use alcohol instead of the other fuel. There is a certain compensation, however, in the fact that alcohol is very easily compressible. In modern internal combustion-engines much of the efficiency is due to the explosive charge which is drawn into the cylinder being compressed into a small space before it is fired. It was the discovery of the value of compressing the gas which made the gas-engine so formidable a rival to the steam-engine, and the wonderful performances of the Diesel engines are due very largely to the fact that the air is compressed in the cylinder to a very high pressure. The jet of oil burns in highly compressed air. And because of the facility with which alcohol can be compressed it is said to be more effective as a source of motive power than would be expected from its comparatively feeble heat. Thus we may sum up the possibilities of the future. Coal, petroleum and their derivatives exist in limited quantities in the world, and so far as we can see the vast drafts which we are taking from them are not being replaced, indeed at this stage of the earth's development cannot be replaced, by any more. Sooner or later we must come to an end of them. Is it not comforting, therefore, to know that there is another source of fuel at hand, inexhaustible, since it can be produced as needed. We have only to set the sun and the ground to work to produce grain, rice, potatoes, or any of the myriad substances which contain starch, and from that, by simple and well-known processes, we can obtain a cheap, safe and reliable fuel. Indeed there seems nothing but the ultimate loss of sunlight, countless millions of years hence, which can ever check the supply of this valuable commodity. What has doubtless, in many cases, been a curse in the past may turn out to be the great boon of the future. CHAPTER IV SOME VALUABLE ELECTRICAL PROCESSES Students of that branch of science known as physics are coming to the conclusion that electricity plays a much more important part in the universe than was supposed. They are led to believe that electrical attraction is the cement which binds together those exceedingly minute particles out of which everything is built up. Whether electricity binds them together or not, it is certain that electrical action can in some cases _separate_ those particles, and this process of separation provides a means of carrying on some very remarkable and useful industrial processes.