More commonly, however, air-gas is made non-automatically, or more or less automatically by the employment of some mechanical means. The light petroleum, benzol, or other suitable volatile hydrocarbon is volatilised, where necessary, by the application of gentle heat, while air is driven over or through it by means of a small motor, which in some cases is a hot-air engine operated by heat supplied by a flame of the air-gas produced. These air-gas producers, or at least the reservoir of volatile hydrocarbon, may be placed in an outbuilding, so that the risk of fire in the house itself is minimised. They require, however, as much attention as an acetylene generator, usually more. It is difficult to give reliable data as to the cost of air-gas, inclusive of the expenses of production. It varies considerably with the description of hydrocarbon employed, and its market price. Air-gas is only slightly inferior hygienically to acetylene, and the colour of its light is that of the incandescent light as produced by coal-gas or acetylene. Air-gas of a certain grade may be used for lighting by flat-flame burners, but it has been available thus for very many years, and has failed to achieve even moderate success. But the advent of the incandescent burner has completely changed its position relatively to most other illuminants, and under certain conditions it seems likely to be the most formidable competitor with acetylene. Since air-gas, and the numerous chemically identical products offered under different proprietary names, is simply atmospheric air more or less loaded with the vapour of a volatile hydrocarbon which is normally liquid, it possesses no definite chemical constitution, but varies in composition according to the design of the generating plant, the atmospheric temperature at the time of preparation, the original degree of volatility of the hydrocarbon, the remaining degree of volatility after the more volatile portions have been vaporised, and the speed at which the air is passed through the carburettor. The illuminating power and the calorific value of air-gas, unless the manufacture is very precisely controlled, are apt to be variable, and the amount of light, emitted, either in self-luminous or in incandescent burners, is somewhat indeterminate. The generating plant must be so constructed that the air cannot at any time be mixed with as much hydrocarbon vapour as constitutes an explosive mixture with it, otherwise the pipes and apparatus will contain a gas which will forthwith explode if it is ignited, _i.e._, if an attempt is made to consume it otherwise than in burners with specially small orifices. The safely permissible mixtures are (1) air with less hydrocarbon vapour than constitutes an explosive mixture, and (2) air with more hydrocarbon vapour than constitutes an explosive mixture. The first of these two mixtures is available for illuminating purposes only with incandescent mantles, and to ensure a