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water capable of being heated from 32 deg. to 212 deg. Fahr. Again, the quantity of heat required to convert a pound of water at 212 deg. Fahr, into steam of the same temperature, is nearly five and a-half (more exactly 5'37) times as much as that requisite to heat a pound of water from the freezing point to the boiling point; therefore the quantities of steam capable of being produced from water at 212 deg. Fahr, by the total heat generated in the combustion of a pound of carbon or of hydrogen, are of course ascertainable by dividing the number of pounds heated from 32 deg. to 312 deg. Fahr, by 5’37. These several quantities are given in the following table :— Quantities of Water Heated, or converted into steam, by the heat generated in combustion of From From From water 40° to 41° F. 32° to 212° F. at 212° F. lbs. lbs. lbs. lbs. 14,500 80’55 15’ 1 carbon. 62,032 344’62 64’2 1 hydrogen. These quantities of 15 pounds and 64'2 pounds of water convertible into steam by the total heat generated in the com bustion of a pound of carbon or of hydrogen, represent what is termed the “ theoretical evaporative powers ” of those sub stances. By the term theoretical, however, it is not to be understood that these values are in any degree imaginary or assumed; they represent actual facts, which have been esta blished as the results of positive observation, and they are theoretical in reference to the practical application, of fuel only in this sense, that these results are not realised in ordinary practice. The reason of this is not the existence of any uncer tainty that the total quantities of heat generated by burning a pound of carbon or a pound of hydrogen are respectively capable of converting 15 pounds and 64'2 pounds of water at 212 deg. Fahr, into steam, but it is simply the fact that, under ordinary circumstances, only a portion of the total heat gene rated in either case is ever available for the production of steam. The statement of the theoretical evaporative power of fuel, or of carbon and hydrogen as constituents of fuel is there fore—like the statement of relative calorific power—only an expression of the relative capabilities, and it indicates in this respect a limit which, though it cannot be exceeded in any case, is never fully attained in practice. In order to ascertain what portion of the heat resulting from the combustion of carbon and hydrogen is available for pro ducing steam, it is necessary to consider what are the condi tions under which fuel is usually burnt, and what becomes of the heat generated in the two cases. In making this inquiry it is also necessary to remember that the several substances con cerned in the combustion of fuel require different quantities of heat to produce equal increments of temperature in equal weights, as stated in the following table:— Quantities of Heat. ’ Carbonic acid gas requires Nitrogen „ Atmospheric air „ Steam „ Water „ . Water at 212° F. „ Heat units. . 2171 . 245 . 238 . 475 .1'000. 966-100 To raise its temperature from T to T x 1° F. for conversion into steam. It will be seen that water has by far the greatest capacity for heat, both in the state of liquid and vapour, and that a very large quantity of heat is rendered latent in the conversion of water into steam. In the combustion of carbon, each pound requires for its conversion into carbonic acid gas, 2'67 pounds of oxygen, which is derived from atmospheric air, and as this contains only 23 per cent, by weight of oxygen, it is necessary to supply about 12 pounds (more accurately 11'61 pounds) of air for every pound of carbon burnt. In the combustion of hydrogen, 8 pounds of oxygen are requisite for each pound of hydrogen, and to furnish this about 35 pounds (more accurately 34’78 pounds) of air must be sup plied. But fuel is never burnt for raising steam in such a way that the supply of air is only just sufficient to furnish oxygen for the conversion of its carbon into carbonic acid gas, and of its hydrogen into water vapour. In order to maintain combustion it is necessary to remove the gaseous products from the fur nace, as well as to supply fresh air continually; and when this is effected, as usual, by the draught of a chimney, the gaseous combustion products become mixed with the fresh air to some extent. The effect of this intermixture would be to retard the combustion of the fuel, if the amount of burnt air or combus tion products in the atmosphere of the furnace exceeded a certain proportion. Consequently, it is necessary to prevent this by supplying more air than would suffice to furnish oxygen for combustion, so as to dilute the combustion products and maintain an excess of oxygen in the atmosphere immediately surrounding the fuel in the furnace. Careful observation has shown that in ordinary boiler furnaces the quantity of air requisite for this purpose amounts to as much as that requisite for effecting the chemical change which takes place in combus tion, so that the total supply of air to such a furnace is usually at the rate of about 24 pounds per pound of carbon burnt, and about 70 pounds per pound of hydrogen burnt. Under ordinary circumstances the relation between the quan tities of these substances burnt as fuel, the total heat generated, the air supply requisite for supporting combustion, and the furnace gas resulting from it will be as follows:— Fuel. Quantity burnt. Air supply. Total heat generated. Furnace gas. Pound. Pounds. Heat units. Pounds. Carbon . . 1 23’22 14,500 24’22 Hydrogen . 1 69’56 62,032 70’56 The heat generated in either case is, at the moment of com bustion, transferred to the gaseous combustion product, and raises its temperature. In the combustion of carbon, the whole of the heat is effective in this way; but in the combustion of hydrogen, a portion of the heat generated is consumed in determining the vaporous condition of the water produced, in the proportion of nine pounds for each pound of hydrogen burnt. As one pound of water at 212 deg. F. requires 966T heat units to convert it into steam of the same temperature, the quantity of heat which becomes latent in this way amounts to 8,694’9 heat units (9x966’1) per pound of hydrogen burnt, or 14 per cent, of the total heat of combustion. That portion of the heat is ineffective, either for increasing the temperature of the combustion product, or for producing steam in the boiler, and it must therefore be deducted from the total heat gene rated, in order to ascertain the amount of heat available, which is as follows, compared with that generated by the combustion of carbon:— Quantity burnt. Total heat generated. Latent heat of water vapour pro duced. Available heat. Equivalent evaporation of water at 212 F. Carbon . . Hydrogen Pound. 1 1 Heat units. 14,500 62,032 Heat units. 8695 Heat units. 14,500 53,337 Pounds. 15 55 In the combustion of carbon, under the conditions above mentioned, the products constituting the furnace gas amount to nearly 25 pounds per pound of carbon burnt, and they require the following quantities of heat to raise their tempera ture one degree of Fahrenheit’s scale:— Specific Heat. Pounds. Heat units. Heat units. Carbonic acid gas . . 3’67 x ’217 = ■79639 Nitrogen . . . . . 8’94 x -245 = 2’19030 Surplus air . . . . 11’61 x -238 = 2’76318 24’22 5’74987 The increase of temperature resulting from the combustion of carbon is therefore found by dividing the number of heat units, representing the total quantity of neat generated, by the number of heat units requisite to raise the temperature of