228 STEAM. SECT. XXV. 



It is found that the absolute quantity of heat consumed 

 in the process of converting water into steam is the same 

 at whatever temperature water may boil, but that the 

 latent heat of steam is always greater exactly in the same 

 proportion as its sensible heat is less. Steam raised at 

 212 under the ordinary pressure of the atmosphere, 

 and steam raised at 180 under half that pressure, con- 

 tain the same quantity of heat, with this difference, that 

 the one has more latent heat and less sensible heat than 

 the other. It is evident that 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 ; and therefore in the 

 steam engine, equal weights of steam at a high pressure 

 and a low pressure are produced by the same quantity 

 of fuel ; and whatever the pressure of the steam may 

 be, the consumption of fuel is proportional to the quan- 

 tity of water converted into vapor. Steam at a high 

 pressure expands as soon as it comes into the air, by 

 which some of its sensible heat becomes latent ; and as 

 it naturally has less sensible heat than steam raised under 

 low pressure, its actual temperature is reduced so much 

 that the hand may be plunged into it without injury the 

 instant it issues from the orifice of a boiler. 



The elasticity or tension of steam, like that of common 

 air, varies inversely as its volume ; that is, when the 

 space it occupies is doubled, its elastic force is reduced 

 one-half. The expansion of steam is indefinite ; the 

 smallest quantity of water when reduced to the form of 

 vapor, will occupy many millions of cubic feet ; a wonder- 

 ful illustration of the minuteness of the ultimate parti- 

 cles of matter ! The latent heat absorbed in the forma- 

 tion of steam is given out again by its condensation. 



Steam is formed throughout the whole mass of a 

 boiling liquid, whereas evaporation takes place only at 

 the free surfaces of liquids, and that under the ordinary 

 temperature and pressure of the atmosphere. There 

 is a constant evaporation from the land and water all 

 over the earth. The rapidity of its formation does not 

 altogether depend upon the dryness of the air ; according 

 to Dr. Dalton's experiments, it depends also on the dif- 

 ference between the tension of the vapor which is form- 



