carbon in the fuel of possibly 4.5 per cent of the total heat in the fuel burned. When this is considered, the importance of a complete flue gas analysis is apparent. Table 34 gives the densities of various gases together with other data that will be of service in gas analysis work. TABLE 34 DENSITY OF GASES AT 32 DEGREES FAHRENHEIT AND ATMOSPHERIC PRESSURE ADAPTED FROM SMITHSONIAN TABLES +----------+----------+--------+---------+----------+---------------+ | | | | | | Relative | | | | | Weight | | Density, | | | | | of | Volume | Hydrogen = 1 | | | |Specific|One Cubic| of +-------+-------+ | Gas | Chemical |Gravity | Foot |One Pound | |Approx-| | | Symbol | Air=1 | Pounds |Cubic Feet| Exact | imate | +----------+----------+--------+---------+----------+-------+-------+ |Oxygen | O | 1.053 | .08922 | 11.208 | 15.87 | 16 | |Nitrogen | N | 0.9673 | .07829 | 12.773 | 13.92 | 14 | |Hydrogen | H | 0.0696 | .005621 | 177.90 | 1.00 | 1 | |Carbon | | | | | | | | Dioxide | CO_{2} | 1.5291 | .12269 | 8.151 | 21.83 | 22 | |Carbon | | | | | | | | Monoxide | CO | 0.9672 | .07807 | 12.809 | 13.89 | 14 | |Methane | CH_{4} | 0.5576 | .04470 | 22.371 | 7.95 | 8 | |Ethane |C_{2}H_{6}| 1.075 | .08379 | 11.935 | 14.91 | 15 | |Acetylene |C_{2}H_{2}| 0.920 | .07254 | 13.785 | 12.91 | 13 | |Sulphur | | | | | | | | Dioxide | SO_{2} | 2.2639 | .17862 | 5.598 | 31.96 | 32 | |Air | ... | 1.0000 | .08071 | 12.390 | ... | ... | +----------+----------+--------+---------+----------+-------+-------+ [Illustration: 1942 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters in the Singer Building, New York City] CLASSIFICATION OF FUELS (WITH PARTICULAR REFERENCE TO COAL) Fuels for steam boilers may be classified as solid, liquid or gaseous. Of the solid fuels, anthracite and bituminous coals are the most common, but in this class must also be included lignite, peat, wood, bagasse and the refuse from certain industrial processes such as sawdust, shavings, tan bark and the like. Straw, corn and coffee husks are utilized in isolated cases. The class of liquid fuels is represented chiefly by petroleum, though coal tar and water-gas tar are used to a limited extent.