fields, generally speaking, are in Scotland, England and the United States. It is found in less quantity, in Germany in the Ruhr district, in Westphalia and Silesia, in the north of France, Austria, Russia, China, Japan, India, Australia, New Zealand and Canada. "Cannel" coal, a variety of bituminous coal, is found in the Midlands UNITED STATES ARMORED CRUISERS "MONTANA" AND "NORTH CAROLINA" of England and in the United States. It is used principally for making illuminating gas and for domestic purposes. The principal lignite fields are in France, Italy, Germany and Austria, but lignite is also found in the United States and in Sweden. The theoretical heating value of fuel is the heat which it develops when consumed under theoretically correct conditions-which are practically only obtained in the laboratory-and it is expressed in heat units or thermal units. In England and the United States of America the British thermal unit is adopted, this being the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit. On the Continent of Europe the "calorie" is used, and the standard is the heat required to raise the temperature of one kilogram of water one degree Centigrade. To convert calories per kilogram of coal into British thermal units per pound of coal, multiply by 1.8. The theoretical heating value of the above-mentioned coals varies between 7000 and 15,500 British thermal units per pound, depending largely on the varying amounts of incombustible matter or ash that the coals contain. The semi-bituminous coals of the Pocahontas and Cardiff varieties are the most uniform in this respect, the ash being only 3 to 8 per cent.; Belgian "Demigras" will run from 5 to 15 per cent., while the residue in Transvaal coal may reach 25 to 35 per cent. The anthracite coals, as mined, contain from 15 to 30 per cent. of refuse or slate. Most of this, however, is usually removed when the coal is prepared for the market, so that anthracite, as sold, may contain as little as 3 per cent. On the other hand, the smaller sizes may run very high in ash, and cases have been known where 50 per cent. refuse has been found in boiler tests. Bituminous coals are extremely variable, running from 5 to 35 per cent. ash, while the percentage in lignite is usually considerably under 10. The heat value of the combustible portion of the coal (ash and moisture deducted) is also quite variable, and depends on the quality of the volatile matter, which may be either very rich in hydrocarbons, as in semi-bituminous coals, or comparatively high in oxygen, as in many of the bituminous coals and lignite. So much, in fact, does the amount of oxygen found in lignite detract from the calorific value of the volatile matter, that the combustible portion of lignite is worth only about threefourths that of semi-bituminous coal. APPROXIMATE CHEMICAL COMPOSITION OF SEVERAL TYPICAL KINDS OF SOLID FUELS The elements in the coal from which we derive heat are carbon in its solid state, hydrogen, and sometimes a little sulphur. The hygroscopic water which it contains is injurious, as it absorbs heat for its own evaporation. The heat value of the fuel may be calculated from the analysis by means of Dulong's formula, as follows: B.T.U. per pound equal 146 C+620 (H-10) + 40 S in which C, H, O and S are, respectively, the percentages of carbon, hydrogen, oxygen and sulphur in the fuel, and the constants are the most recent average heat values for carbon, hydrogen and sulphur, each divided by 100. The actual heating value of a coal, as determined by test with an instrument known as a "bomb calorimeter" (see page 71), agrees very closely with that calculated from the analysis, usually within 2 per cent., when both the analysis and the calorimeter test are made by a skilled chemist. The analyses given in the foregoing table are called "ultimate analyses," since the constituents of the fuel, except the moisture and ash, are reduced to the ultimate chemical elements. Another kind of analysis, called "proximate analysis," is more commonly used, which separates the coal into four parts, viz.: moisture, volatile matter, fixed carbon and ash. The proximate analysis is of great value for indicating the general character of a coal. By dividing the percentages of volatile matter and fixed carbon each by their sum, we obtain the percentages of each in the "combustible," or coal dry and free from ash. These percentages serve to identify the class to which the coal belongs, as follows: These various kinds of coal act very differently during their combustion in a furnace, and to get the best results each must be handled in the way best suited to its characteristics; and the size and design of the furnace must also be adapted to the particular requirements of the coal. With anthracite coal disintegration and distillation take place very slowly, with semi-bituminous coal they take place somewhat faster, and |