_Answer._ The heat given out by the combustion of one pound of the chemical elements of which coal is composed has been determined by experiment, and from such data, knowing the substances of which fuel is composed, we can determine the amount of heat which would be developed if they were each perfectly consumed. Thus the total heat of combustion of one pound of hydrogen is 62,032 units, and of the same quantity of carbon 14,500 units.[97] Therefore, if a pound of coal contains 5 per cent. of hydrogen, the heat given out by the combustion of that element will be 62,032 × 0.05 = 3,101.60 units, and if it has 80 per cent. of carbon, the combustion of the latter would develop 14,500 × 0.80 = 11,600 units, so that the total heat of the combustion of these two elements would be 3,101.6 + 11,600 = 14,701.6 units. It was shown in answer to Question 40 that it required 1,213.4 units of heat to convert water at zero to steam of 100 pounds pressure. As steam is usually generated from water at a temperature of about 60 degrees, the total heat required to convert it into steam of 100 pounds pressure would be 1,213.4 - 60 = 1,153.4 units. A pound of average bituminous coal, therefore, contains heat enough to convert 12¹⁄₄ lbs. of water into steam of 100 lbs. absolute pressure. Ordinarily only about half that amount of water is evaporated in locomotive boilers per pound of fuel. [97] The experiments which have been made to determine these amounts do not agree exactly, but those given are thought to be the most trustworthy. QUESTION 399. _What are the chief causes of this waste of heat?_ _Answer._ It is due, _first_, to the waste of unburnt fuel in the solid state. This occurs when fuel which is very fine falls through the grates, or is carried through the tubes and out of the stack in the form of cinders.[98] [98] It should be remarked here that some and perhaps most of the cinders which are carried out of the stack are not combustible but are composed of the same materials that form clinkers on the grate. _Second_, to the waste of unburnt fuel in the gaseous or smoky state. The method of preventing this waste by a sufficient supply and proper distribution of air has been explained in the answer to preceding questions.