Pulverized Fuels--Considerable experimental work has been done with pulverized coal, utilizing either coal dust or pulverizing such coal as is too small to be burned in other ways. If satisfactorily fed to the furnace, it would appear to have several advantages. The dust burned in suspension would be more completely consumed than is the case with the solid coals, the production of smoke would be minimized, and the process would admit of an adjustment of the air supply to a point very close to the amount theoretically required. This is due to the fact that in burning there is an intimate mixture of the air and fuel. The principal objections have been in the inability to introduce the pulverized fuel into the furnace uniformly, the difficulty of reducing the fuel to the same degree of fineness, liability of explosion in the furnace due to improper mixture with the air, and the decreased capacity and efficiency resulting from the difficulty of keeping tube surfaces clean. Pressed Fuels--In this class are those composed of the dust of some suitable combustible, pressed and cemented together by a substance possessing binding and in most cases inflammable properties. Such fuels, known as briquettes, are extensively used in foreign countries and consist of carbon or soft coal, too small to be burned in the ordinary way, mixed usually with pitch or coal tar. Much experimenting has been done in this country in briquetting fuels, the government having taken an active interest in the question, but as yet this class of fuel has not come into common use as the cost and difficulty of manufacture and handling have made it impossible to place it in the market at a price to successfully compete with coal. Coke is a porous product consisting almost entirely of carbon remaining after certain manufacturing processes have distilled off the hydrocarbon gases of the fuel used. It is produced, first, from gas coal distilled in gas retorts; second, from gas or ordinary bituminous coals burned in special furnaces called coke ovens; and third, from petroleum by carrying the distillation of the residuum to a red heat. Coke is a smokeless fuel. It readily absorbs moisture from the atmosphere and if not kept under cover its moisture content may be as much as 20 per cent of its own weight. Gas-house coke is generally softer and more porous than oven coke, ignites more readily, and requires less draft for its combustion. [Illustration: 16,000 Horse-power Installation of Babcock & Wilcox Boilers and Superheaters at the Brunot's Island Plant of the Duquesne Light Co., Pittsburgh, Pa.] THE DETERMINATION OF HEATING VALUES OF FUELS The heating value of a fuel may be determined either by a calculation from a chemical analysis or by burning a sample in a calorimeter.