262 STEAM. [SECT. xxv. 



180 under half that pressure, contain 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 consump- 

 tion of fuel is proportional to the quantity of water converted 

 into vapour. Steam at a high pressure expands as soon as it 

 comes into the air, by which some of its sensible heat be- 

 comes 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 with- 

 out 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 vapour, will occupy 

 many millions of cubic feet ; a wonderful illustration of the 

 minuteness of the ultimate particles of matter ! The latent 

 heat absorbed in the formation 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 sur- 

 faces 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 difference between the tension of the vapour 

 which is forming and that which is already in the atmosphere. 

 In calm weather, vapour accumulates in the stratum of air 



