But while this method of utilizing motor spirit for illuminating purposes has been adopted to a very large extent by means of portable lamps, a greater field has been developed both in this and other countries in connection with the domestic and industrial use of petrol air-gas for lighting purposes. These apparatus in the main possess but slight differences. The essential principle of each is that motor spirit is carburetted and then, in the form of an oil gas, conducted through pipes in the same manner as coal gas is burned, to the rooms in which it is required. The carburetted air-gas is automatically produced, and the small cost at which these automatic plants can be supplied has rendered this system of lighting deservedly popular. Its great economy also is an important point, for 1 gallon of motor spirit will yield almost 30 cubic feet of vapour. This vapour, in order to form a lighting agent, is mixed to the extent of over 98 per cent. air and less than 2 per cent. petrol vapour, so that 1 gallon of motor spirit will produce, approximately, 1,500 cubic feet of air-gas. The plants, which are usually worked by a small hot-air engine (or, alternately, by the use of weights), supply only the demand created, and their control is automatic perfection. [Illustration: THE ANGLO-AMERICAN OIL COMPANY’S OIL COOKER] To-day, petroleum plays quite an important part in heating arrangements, and several stoves are upon the market which burn the ordinary illuminating oil. The prettily designed heating stoves of the “Perfection” or “Reform” make are largely in use, the efficacy and economy of these being responsible for their popularity. Various makes of oil cookers are also in large demand. These range from the small variety like the “Primus” stove, which burns illuminating oil under pressure, to the oil cooking stoves of the Anglo-American Oil Company, Ltd., which are quite competent to meet the requirements of practically any household. These latter stoves consume illuminating oil by means of the circular wick arrangement, and are in several sizes, one of the best being that containing three lighters. Two of these are under the oven, and one at the end can be used for boiling purposes. Speaking from several years’ experience of these stoves, I can say that they are truly perfection. They are very economical, are easily cleaned, and when in full operation give off not the slightest odour. The oven is more readily heated than with the coal gas apparatus, and the properly diffused heat cooks all kinds of food most readily and perfectly. The illustration of the stove given on the preceding page will afford the reader a good idea of the apparatus, which deserves to be even more popular than it is at present. CHAPTER VIII INTERNAL COMBUSTION ENGINES