233. All burners which are designed to produce heat rather than light, are constructed to mingle the gas with more or less air before burning. They, therefore, belong to the class known as atmospheric burners. There are two varieties of these burners now in common use-the Bunsen and the Fletcher burner.

234. The Bunsen burner, shown in Fig. 88, is named after its inventor.

Fig. 88.

It consists of a gas tube a which projects part way into a large tube b, called the mixing tube. Air is admitted through holes c, which are closed or regulated by means of the collar or slide d. The gas issuing from a and the streams of air from the holes c mingle in the upper part of the tube b and form a mixture which will burn over the mouth of the tube. The flame will be quite large and unsteady, and if illuminating gas be burned, it will show a pale blue color with a tendency to green. If the proportion of air is sufficient to make the mixture combustible without the aid of the atmosphere, the flame will flash back with a sharp puff or explosion, and will then burn at the orifice of a with an ordinary yellow smoky flame. To prevent this, the slide d must be adjusted so as to restrict the supply of air just below the explosion limit. If the air supply is much too large, the explosion may be sufficiently violent to extinguish the flame.

235. In the Fletcher, or solid flame, burner, the top of the chamber is covered with stout wire gauze, and the gas burns as it issues through the meshes of the gauze. By this arrangement, the gas is provided with nearly or quite all of the oxygen that it requires for combustion, and, consequently, burns close to the gauze with a small compact flame of great intensity. The color of the flame, when using ordinary illuminating gas, is a bright green with a few traces of blue. The Fletcher burner is able to develop a higher heat from the gas than the Bunsen burner, because it permits a larger proportion of air to be mixed with the gas, the flame being prevented from blowing back by the wire gauze.

236. The variety of burner commonly employed in cooking stoves is constructed on the Bunsen-burner principle. Cooking-stove burners are frequently made with two or more rings of jets. Each ring should have its own mixing tube and gas-cock, so that one or all of them may be used as desired.

The best service can be had from an atmospheric burner by regulating both the air and gas supply at the same time. For this purpose compound cocks are now made which control both inlets simultaneously.

Gas stoves, gas grates, gas logs, and water heaters are all heated by means of atmospheric burners.

237. A gas log is shown in Fig. 89. The log is made of fireclay, and is perforated with a large number of small orifices, through which the mingled gas and air, or the gas only, passes out and burns. The log is hollow, and its interior serves as a chamber in which the gas and air are mixed before combustion. The heat is radiated directly from the small flames which nearly cover the surface of the log. The valve which controls the gas supply is located under the floor, the handle being a little above the floor, as shown.

Fig. 89.