But that is somewhat of a digression. This dissertation on organic chemistry was simply intended to lead up to the question of liquid fuels, all of which are organic.
In the life of to-day one of the most important things is petroleum. This is a kind of liquid coal. Just how it was formed down in the depths of the earth is not clear. One idea is that it is due to the decomposition of animal and vegetable matter. Another is that certain volcanic rocks which are known to contain carbide of iron might, under the influence of steam, have in bygone ages given off petroleum, or paraffin, to use the other name for the same thing.
In many parts of the world these deposits of oil are obtained by sinking wells and pumping up the oil. In others the liquid gushes out without the necessity of pumping at all. This is believed to be due to the fact that water pressure is at work. Artesian wells, from which the water rushes of its own accord, are quite familiar, and are due to the fact that some underground reservoir tapped by the well is fed through natural pipes, really fissures in the rock, from some point higher than the mouth of the well. Now supposing that a reservoir of oil were also in communication with the upper world in the same way, the descending water would go to the bottom, underneath the lighter oil, and would thus lift it up, so that on being tapped the oil would rush out.
Another source of mineral oil is shale, such as is to be found in vast deposits in the south-east of Scotland. This shale is mined much as coal is: it is then heated in retorts as coal is heated at the gas-works: and the vapour which is given off, on being condensed, forms a liquid like crude petroleum.
In all these cases the original oil is a mixture of a great number of grades differing from each other in various ways. They are all "hydro-carbons," which means compounds of carbon and hydrogen, and they extend from cymogene (the molecules of which contain four atoms of carbon and ten of hydrogen) to paraffin wax, which has somewhere about thirty-two of carbon to sixty-six of hydrogen. For practical purposes their most important difference is the temperature at which they boil, or turn quickly into vapour.
This forms the means by which they are sorted out. In a huge still, like a steam-boiler, the crude or mixed oil is gradually heated, and the gas given off is led to a cooling vessel where it is chilled back into liquid. The lightest of all, cymogene, is given off even at the freezing-point of water. That is led into one chamber and condensed there. Then, as the temperature rises to 18° C., rhigolene is given off: that is collected and condensed in another vessel. Between 70° and 120° petroleum ether and petroleum naphtha are produced, and they together constitute what is commonly called petrol. Between 120° and 150° petroleum benzine arises. All the foregoing taken together constitute about 8 to 10 per cent. of the whole crude oil. Then between 150° and 300° there comes off the great bulk of the oil, nearly 80 per cent., the kerosene or paraffin which we burn in lamps. Above 300° there is obtained another oil, which is used for lubrication, also the invaluable vaseline, and finally, when the still is allowed to cool, there remains a solid residuum known as paraffin wax. This process is known as fractional distillation, and it will be noticed that it consists essentially in collecting and liquefying separately those vapours which are given off at different ranges of temperature. For our purpose in this chapter we are mainly concerned with the petrol and the kerosene.
Many efforts have been made in times gone by to use kerosene for firing the boilers of steam-engines. In naval vessels a great deal is so used at the present time. But the chief method of employing oil for generating power is to use it in an internal combustion-engine. These machines have been dealt with at length in Engineering of To-day and Mechanical Inventions of To-day and so must be simply mentioned here. They consist of two types. In one, which is exemplified by the ordinary car or bicycle motor, the oil is gasified in a vessel called a carburetter or vaporiser and then led into the cylinder of the engine, together with the necessary air to enable it to burn. At the right moment a spark ignites the mixture, which burns suddenly, causing a sudden expansion, in other words, an explosion. Thus the power of the engine is derived from a succession of explosions. If the fuel be petrol it vaporises at the ordinary temperature of the engine and needs no added heat. With kerosene, however, heat has to be employed in the vaporiser to make it turn readily into a gas.
The other method is employed in engines of the new "Diesel" type, in which the cylinder of the engine, being already filled with hot air, has a jet of oil sprayed into it. The heat of the air causes it to burst into flame, causing an expansion which drives the engine.
An important feature in the latter type of engine is that the oil is very completely burnt, so that very heavy oils can be used, oils which, if employed in an engine of the other kind, would choke up the cylinder with soot. In other words, the range of oils which can be used in this new kind of engine is much wider than is possible in the others. The latter may be likened to a fastidious man who is very particular about his food, while the former resembles the man of hearty appetite who can eat anything. And just as a man of the latter sort is more easily provided for by the domestic authorities, so the Diesel engine makes the problem of the provision of liquid fuel much simpler.
For it must never be forgotten that the provision of liquid fuel for the world is by no means a simple matter, since the supply is by no means adequate. The output runs into thousands of millions of gallons, and the whole world is being searched for new fields of oil, and yet it is all swallowed up as fast as it can be produced, while the coal mines do not feel the competition. A year or so ago the United States and Russia between them (and they are the greatest producers) obtained 5,000,000,000 gallons of oil, seemingly an enormous quantity. But, on the other hand, Great Britain alone produces over 250,000,000 tons of coal per annum. If, therefore, liquid fuel is to displace coal, as some people lightly think it is going to do, the supply will have to be multiplied many times. In the amount of heat which it is capable of giving the coal of Great Britain alone beats the oil produced by the whole world.