14 Prime Movers. 



machine the amount of heat expended in work approaches the 

 nearest to equality with the entire quantity. 



The same substance, in the solid state, has less capacity for 

 heat than in the liquid state ; and, in the liquid, less than in the 

 vaporous state ; but combustion may change a solid, or a liquid, 

 into a gas which has a less capacity for heat ; and the heat given 

 out by fuel is the difference between its specific heat and that 

 of the products of its combustion. Charcoal, when burned, 

 takes oxygen from the atmospheric air: and, as that gas is 

 rendered more dense, without alteration of its volume or 

 elastic force, it loses the difference between its specific heat 

 in its former and its latter state. It was found, as the 

 mean of several experiments, that one pound of hydrogen, 

 combining with oxygen, is capable of raising 51,146 pounds 

 of water, one degree Fahrenheit ; one pound of carbon, 

 14,500 pounds of water; one pound of phosphorus, 11,900 

 pounds of water; and one pound of sulphur, 2800 pounds 

 of water. But hydrogen, during combustion, combines with 

 eight times its weight of oxygen; carbon, with only twice 

 and two-thirds of its weight ; phosphorus, with only about 

 once and a quarter its weight ; and sulphur, with only an equal 

 weight. The heat evolved has, therefore, a very close relation 

 to the amount of oxygen which enters into combination. We may 

 deduce the heating power of any kind of fuel from these quanti- 

 ties, if we know the nature and relative amounts of its con- 

 stituents ; since all fuel consists chiefly, or altogether, of carbon, 

 of carbon and hydrogen, or of carbon, hydrogen, and oxygen. 

 There is a loss of heat, by vaporization of water, whenever that 

 fluid is formed during combustion; and this loss is estimated 

 at one-fifth of the power of the hydrogen — any water, mecha- 

 nically present in the fuel, would, of course, be a cause of further 

 diminution. The heating powers above mentioned suppose the 

 combustion to be complete ; but if, from any cause, it is imper- 

 fect, the carbon may be only partially consumed — dense smoke 

 being generated ; or it may combine with only half the quantity 

 of oxygen ; in which case the thermal unit falls from 14,500 to 

 4400 pounds of water. Hydrogen unites with pure oxygen at 

 800°, and burns in the atmosphere at 950° ; carbon unites with 

 pure oxygen at 700°, and burns in the atmosphere at 800°; but 

 when the fuel contains incombustible gases, higher heats are 

 required; and if the temperature should exceed 1200% particu- 

 larly if it approach 1500°, carbon, combining with the earthy 

 matters found in ordinary coal, will form clinkers, and fuel will 

 be wasted. 



The heat of all fuel is that which has been received from the 

 sun, duriug the chemical changes which take place in the 

 growth of those plants that constitute not only the forests at 



