~Natural gas.~ This substance, so abundant in many localities, varies much in composition, but is composed principally of methane. When used for lighting purposes it is usually burned in a burner resembling an open Bunsen, the illumination being furnished by an incandescent mantle. This is the case in the familiar Welsbach burner. Contrary to statements frequently made, natural gas contains no free hydrogen. TABLE SHOWING COMPOSITION OF GASES =====================+================+========+========+========== | PENNSYLVANIA | COAL | WATER | ENRICHED | NATURAL | GAS | GAS | WATER | GAS | | | GAS ---------------------+----------------+--------+--------+---------- Hydrogen | | 41.3 | 52.88 | 30.00 Methane | 90.64 | 43.6 | 2.16 | 24.00 Illuminants | | 3.9 | | 12.05 Carbon monoxide | | 6.4 | 36.80 | 29.00 Carbon dioxide | 0.30 | 2.0 | 3.47 | 0.30 Nitrogen | 9.06 | 1.2 | 4.69 | 2.50 Oxygen | | 0.3 | | 1.50 Hydrocarbon vapors | | 1.5 | | 1.50 =====================+================+========+========+========== These are analyses of actual samples, and may be taken as about the average for the various kinds of gases. Any one of these may vary considerably. The nitrogen and oxygen in most cases is due to a slight admixture of air which is difficult to exclude entirely in the manufacture and handling of gases. ~Fuels.~ A variety of substances are used as fuels, the most important of them being wood, coal, and the various gases mentioned above. Wood consists mainly of compounds of carbon, hydrogen, and oxygen. The composition of coal and the fuel gases has been given. Since these fuels are composed principally of carbon and hydrogen or their compounds, the chief products of combustion are carbon dioxide and water. The practice of heating rooms with portable gas or oil stoves with no provision for removing the products of combustion is to be condemned, since the carbon dioxide is generated in sufficient quantities to render the air unfit for breathing. Rooms so heated also become very damp from the large amount of water vapor formed in the combustion, and which in cold weather condenses on the window glass, causing the glass to "sweat." Both coal and wood contain a certain amount of mineral substances which constitute the ashes. ~The electric furnace.~ In recent years electric furnaces have come into wide use in operations requiring a very high temperature. Temperatures as high as 3500 deg. can be easily reached, whereas the hottest oxyhydrogen flame is not much above 2000 deg.. These furnaces are constructed on one of two general principles. [Illustration: Fig. 67]