504 FUEL 



tho steam-jet is the mechanical action to which I have frequently referred the 

 thorough mixing of the gaseous fuel with the oxygen required for tho purpose of com- 

 bustion tho churning of the particles all together, so that they shall be brought 

 thoroughly into contact. Thero may bo another mode of operation of steam a 

 chemical action whoso effect is to prevent tho deposition of unconsumed carbon from 

 gaseous fuel. All hydrocarbons have a tendency when raised to a high temperature 

 to deposit tho carbon, unless it be instantly burned. We see that in the smoke that 

 a flame tends to give out, if there is anything to prevent immediate and free access of 

 oxygen to the carbon. Now it is possible that besides its mechanical action, the steam- 

 jet may also have a chemical action of tho following kind: The oxygen of tho steam 

 combines with the carbon of tho hydrocarbon fuel, and the hydrogen of both is net 

 free ; there is a mixture thus produced of carbonic oxido and of hydrogen, which is 

 sure to be completely burnt as soon as a sufficient quantity of air gets access to it, and 

 thus the deposit of unconsumed carbon is entirely prevented.' 



Dr. Paul, in a paper communicated to tho Society of Arts, has the following valuable 

 remarks upon the combustion of mineral oils : 



' The volumes of the air-supply and combustion-products for the extreme cases of 

 carbon and hydrogen are as follow : 



' In the combustion of carbon there is no expansion of volume in tho combustion-pro- 

 duct, except that due to the heat generated, which would render the volume at the 

 temperature of combustion (2,522 F.) rather more than six times that of the air sup- 

 plied. By the transfer of heat to the boiler, to such an extent as .to reduce tho tem- 

 perature to 660 F., the volume would be reduced again to about 630 cubic feet per 

 pound of carbon burnt. 



' In the combustion of hydrogen the supply of air required is about three times as 

 largo as that required in the combustion of an equal weight of carbon. There is also 

 an expansion of the combustion-products, independent of the heat generated, and 

 amounting to one-half the normal volume of the hydrogen burnt. The expansion duo 

 to heat is also greater than in the combustion of carbon, on account of the greater 

 amount of heat generated, so that the volume of the furnace-gas at the temperature of 

 combustion (2,791 F.) would be about 6 times that of the air supplied, and tho 

 volume of gas discharged into the chimney would be aboiit 3 times as great as in the 

 combustion of an equal weight of carbon. This larger quantity of gas will, however, 

 contain nearly 4 times as nmch effective heat as that resulting from the combustion of 

 an equal weight of carbon, and its temperature will be about 270 higher, so that in 

 this respect the use of fuel containing a large amount of hydrogen, provided it can be 

 perfectly and readily burnt, presents an advantage as compared with fuel consisting 

 almost entirely of carbon. Rather more than one-fourth of a pound of hydrogen would 

 give as much effective heat as one pound of carbon with a somewhat smaller volume 

 of combustion-products. The extent to which this advantage affects tho value or 

 efficiency of fuel will, of course, depend on the amount of hydrogen it contains. Since 

 no hydrocarbon available as fuel contains more than 15 per cent, of hydrogen, the 

 actual evaporative efficacy of such a material, when used under tho ordinary condi- 

 tions, cannot, at the utmost, be more than about 40 per cent, greater than that of an 

 equal weight of carbon. This, assuming it to be perfectly burnt, and the arrangement 

 of boiler flues or tubes, &c., to be favourable for the transfer of heat, is the maximum 

 effect to be looked for according to the data already given. 



1 The amount of hydrogen in petroleum is probably larger than in any of the other 

 hydrocarbons proposed to bo used as fuel, and that contains, on the average, about 13 

 per cent. In coal and shale oil tho amount of hydrogen is less. Consequently, the 

 evaporative efficacy of these materials, as compared -uith carbon, would not reach tho 

 above limit of 40 per cent, in excess. The ratio Let \vocn these materials and ordi- 

 narily good roal is much about the same in regard to evaporative efficacy, since the 

 hydrogen contained in coal compensates for the oxygon and ash it contains, unless tho 

 amount of these is very considerable. 



'The following tables show the relation between th'- total heat of combust inn and 

 tho available heat of hydrocarbons, containing respectively 14 and 2o percent, of 

 hydrogen, also the amounts of heat consumed, in the furnace-gas, and the mode in 

 which it is disposed of: 



