278 FORCE OF STEAM. SECT. XXVII. 



pressure of the atmosphere, and steam raised at 180 under half 

 the pressure, contain the same quantity of heat, with this 

 difference, that the one has more absorbed heat and less sensible 

 heat than the other. It is evident that, as the same quantity of 

 heat is requisite for converting a given weight of water into 

 steam, at whatever temperature or under whatever pressure the 

 water may be boiled, therefore, in the steam engine, equal 

 weights of steam at a high pressure and a low pressure are pro- 

 duced by the same quantity of fuel ; and whatever the pressure 

 of the steam may be, the consumption of fuel is proportional to 

 the quantity of water converted into vapour. Steam of whatever 

 tension expands on being set free, but the expansion of high 

 pressure steam at the expense of its sensible heat is so great, 

 that the hand may be plunged into it without injury the instant 

 it issues from -the orifice of a boiler. The steam becomes hotter 

 by friction in issuing through the orifice which maintains it in its 

 dry form, for there is no doubt that high-pressure steam is dry. 



The elasticity or tension of steam, like that of common air, 

 varies inversely as its volume that is, when the space it occu- 

 pies is doubled, its elastic force is reduced to one half. The 

 expansion of steam is indefinite ; the smallest quantity of water 

 expanded into vapour will occupy many millions of cubic feet ; 

 a wonderful illustration of the minuteness of the ultimate parti- 

 cles of matter. 



The force of steam, tremendous as the lightning itself when 

 uncontrolled, is merely the result of chemical affinity : it is the 

 chemical attraction between the particles of carbon, of coal or 

 wood, and the oxygen of the atmosphere. Mr. Joule has ascer- 

 tained that a pound of the best coal when burnt gives sufficient 

 heat to raise the temperature of 8086 pounds of water one degree 

 of the Centigrade thermometer, whence it has been computed by 

 M. Helmholtz that the chemical force arising from the combus- 

 tion of that pound of coal is capable of lifting a body of one 

 hundred pounds weight to the height of twenty miles. That is 

 the work performed by the heat arising from the combustion of 

 a pound of coal. In all cases where work is produced by heat, a 

 quantity of heat proportional to the work done is expended ; and 

 conversely, by the expenditure of a like quantity of work, the 

 same amount of heat may be produced. The equivalence of heat 

 and work is a law of nature. The mechanical force exerted by 



