It remains, therefore, to cool and condense these vapours. This is accomplished by an apparatus called a “condenser” which is connected to each still. An elementary condenser consists merely of a coil of pipe submerged in a tank of cold water. The vapour leaving the still passes through the submerged coils in which the vapour by cooling is caused to return to a liquid condition. Into one end of the condenser coil, therefore, the vapour from the still enters and from the other end there flows the condensed liquid. The first and most important step in the process of refining all crude petroleum is conducted in the fashion above described. A refinery of large size will have perhaps 100 of those crude stills which are generally arranged in groups or batteries, each battery containing a dozen or more stills. From each still the condenser pipes are led to a “receiving house” which is located in some central position. In this manner it becomes possible for a single responsible supervisor to observe and control the operation of a large number of stills. The supervisor is called the “stillsman” and upon him rests the responsibility for directing the initial process of separation or sifting by which the crude petroleum received at the refinery is roughly separated into different “fractions” or parts, each of which by further refining becomes a marketable petroleum product. As generally conducted, this first distillation process separates the crude petroleum oil into four major fractions. The fraction which has the lowest boiling point and is therefore the first to be driven off from the crude petroleum in the still as the latter is heated, is the naphtha or gasoline fraction. When all the naphtha or gasoline from any particular still has been driven off, the stillsman, stationed in the receiving house and able to observe constantly the character of the condensed liquid, which is delivered by the pipe from the condenser coil to the house, will change the connections in the receiving house so that the next “distillate” to be received will flow to a separate tank. This second distillate which comes into the receiving house and is thus diverted to a separate tank will be the illuminating oil distillate or, in refinery parlance, the “refined oil distillate.” It is interesting to note that “refined oil” to a petroleum refiner still means kerosene illuminating oil, since in the original petroleum industry this illuminating product was the only fraction of the crude oil which was highly purified or refined. The entire remainder of the crude petroleum, including gasoline and the lubricating oils and other products heavier than kerosene, were either discarded wholly or else sold for whatever they would bring in an unrefined or very poorly refined condition.