come latent. In fact, the steam would then be identical in its constitution and 

 properties with steam raised from water at the temperature of 212°, and under 

 the ordinary atmospheric pressure. If the piston be raised or lowered under 

 these circumstances, the steam would take all possible temperatures and pres- 

 sures, and would, in each case, be identical with the steam raised from water 

 under a corresponding pressure and temperature. 



The sum of the latent and sensible heats of steam being always the same, 

 it follows that, if we know the latent heat of steam at any one temperature, 

 the latent heats at all other temperatures is a subject of easy calculation. 

 Thus, if the sum of the latent and sensible heats be 1,212°, the latent heat of 

 steam at 500° of temperature must necessarily be 712°, and steam at the tem- 

 perature of 1,000° will have only 212° of latent heat. 



It follows also that, in order to maintain water in a state of vapor, the sum 

 of its latent and sensible heats cannot be less than 1,212°; and if it be re- 

 duced below this, by being caused to impart heat to any other object, then a 

 portion of the vapor must return to the liquid state, giving its latent heat to the 

 vapor which remains, so as to raise the sum of the latent and sensible heats 

 of that vapor to 1,212°. When so much steam becomes liquid as is capable 

 of accomplishing this, then the remainder of the vapor will continue in the 

 aeriform state. If steam receives no heat except that which is imparted to 

 the water during the process of vaporization, the sum of its latent and sensible 

 heats cannot be greater than 1,212°, and therefore such steam cannot lose any 

 heat without undergoing partially the process of condensation ; but if steam, 

 after the process of vaporization, has received an increase of temperature by 

 heat supplied from some external source, then the sum of its latent and sensi- 

 ble heats will be greater than 1,212° by the heat so received, and the steam 

 may lose that excess of heat above 1,212° without undergoing any condensa- 

 tion. 



In considering the properties of steam at present, we shall, however, regard 

 it as having received no heat except that which it receives in the process of 

 vaporization, unless the contrary be distinctly expressed. 



It is well known that air and the gases generally admit of compression and 

 rarefaction without any practical limit, and that their elasticity is susceptible 

 of increase and diminution, as the space they fill is contracted or enlarged. 

 Let a cylinder, in which a piston moves air-tight, have the space below the 

 piston filled with atmospheric air in its ordinary state. By the application of 

 adequate mechanical force, the piston may be pressed toward the bottom of 

 the cylinder, so that the air beneath it shall be forced into a more confined 

 space. The effect of this compression will be twofold — an increase of tem- 

 perature and an increase of elasticity. If the piston, on the other hand, be 

 raised so as to allow the air to expand into a more enlarged space, the contrary 

 effects will ensue — the temperature of the air will fall, and its elasticity will 

 be diminished. Whether air thus enclosed be compressed into a more limited 

 space, or allowed to expand into a more enlarged space, it never passes from 

 the aeriform state, nor loses its property of elasticity. No known degree of 

 compression has caused it to become a liquid, nor has any degree of expansion 

 caused it to lose its elastic property. 



Let us now suppose the space below the piston, instead of air, to be filled 

 with steam raised from water at the temperature of 212°. If the piston be 

 raised, this steam will expand, its temperature will fall, and its elastic force 

 will diminish in the same manner as already described for common air, and, 

 as with common air, there is no known limit to the extent of this expansion. 



If, however, the piston be pressed toward the bottom of the cylinder, it has 

 been generally stated that steam will not comport itself like common air under 



