The crude petroleum oil is a liquid and cannot be sifted on screens as is the crude gravel, but nature has given it properties in consequence of which it may be separated into its constituents almost as easily as is the gravel. These properties are the different boiling points of the several constituents. Thus, when water or any other single liquid is heated it continues to increase in temperature until boiling begins, after which its temperature remains the same, no matter how rapidly the heat is applied, until all of the liquid has been boiled away. When petroleum is heated, however, it begins to boil at a very low temperature, a temperature hardly hot enough to injure the skin, in some cases. It is not the whole of the petroleum which is boiling, however, but only the very lightest part of it, that is, the gasoline or naphtha. If the temperature were to be held constant for a short length of time all of the gasoline would have been boiled off, and although the liquid would be just as hot as it was before, the boiling would cease entirely. If the heating is now continued, however, and the temperature of the oil raised to some higher figure, it again begins to boil and now it is the kerosene constituent of the crude petroleum which is being converted into vapour and driven out of the liquid. After a time all of this kerosene will be gone, and as before, the liquid, although still at the same temperature at which it has just previously been actively boiling, remains quiescent. In this fashion the various constituents of the crude petroleum may be separated from one another by a “sifting” operation somewhat similar to that used to separate sand from gravel and gravel from rock, except that instead of employing screens to effect the separation there is employed an apparatus in which the heat of the oil can be gradually increased and the products, which are successively driven off in this fashion, separated from one another. The apparatus commonly employed for this purpose is called a “still” and consists merely of a steel receptacle, usually in the form of a horizontal cylinder, much like a simple steam boiler. These stills have been developed to large capacity, some of them holding upwards of 50,000 wine gallons of oil at one time. The still is mounted over a furnace which is usually heated by coal just as an ordinary steam boiler. In this still the temperature of the crude petroleum is gradually raised and with each elevation in temperature a different product is boiled or driven off the mass of liquid until finally nothing remains in the still except a small quantity of black residue which is known as petroleum coke.