The most practical methods which have been devised for combining the purity of air in a room with artificial light produced from ordinary coal gas, may be classed under four heads:-- (1) The sun burner, in which the products of combustion are removed rapidly from contact with the air of the room. (2) The globe light, in which the fresh air is supplied and the products of combustion are removed to the outside without any contact with the air of the room. (3) The regenerative gas light. (4) The incandescent gas light. Their several merits are thus discussed in one of the Health Exhibition Handbooks. The sun burner is practically a powerful ventilator, which, by means of the great heat generated, draws a large volume of air away with the fumes of the gas; it thus relieves the air of the room of the impurities caused by combustion, and at the same time removes impurities generated from other causes. This burner is indeed a sufficiently powerful ventilator to continue acting even in the face of the counteracting draught of an open fireplace; and is consequently much used for crowded rooms. For this dual purpose, it requires to have its fumes carried up through a straight vertical tube direct to the open air. This burner is made by Strode & Co., 67 St. Paul’s Churchyard, and shown in Fig. 55. [Illustration: 55. The Sun Burner.] The globe light has been designed to prevent the products of combustion from mingling at all with the air of a room, but it does not provide for the ventilation of the room at the same time. The principle of the best form is that it should be burned in a glass globe separated from the air of the room; that is to say, the air required for supporting combustion is brought into the globe from the outer air, and the products of combustion are carried away into the outer air without mixing with the air of the room. This light, like the sunlight, is limited in its application. It can be placed near an outside wall, or in a room directly under a roof. If fed with fresh air from the room itself, and if a fire-proof flue be constructed in the ceiling leading into a vertical flue, this light can be put in any part of a room; but the draught from the open fire would be very likely to draw the products of combustion back into the room. This is also made by Strode & Co.