LA W OF THE EXPANSION OF STEAM. 255 



density will diminish, when the volume is increased, as in 

 permanent gases; and if the volume be again diminished, 

 the pressure and density will increase, until they return to 

 the maximum due to the temperature ; and the effect of any 

 further diminution of volume, or attempt to further in- 

 crease the density at the same temperature, is simply 

 accompanied by the precipitation of a portion of the vapor 

 to the liquid state, the density remaining the same. 



On the contrary, if the application of heat be continued 

 when all the liquid is evaporated, the state of saturation 

 ceases, and the temperature and pressure are increased, 

 while the density remains the same ; the steam is said to be 

 superheated, or surcharged with heat, and it becomes more 

 perfectly gaseous. While in this condition, if it were to be 

 replaced in contact with water of the original temperature, 

 it would evaporate a part of the water, transferring to it 

 the surcharge of heat, and would resume its normal state of 

 saturation. 



If the space for steam over the water remain unaltered, 

 then, if the temperature is raised by the addition of heat, 

 the density of the vapor is increased by fresh vaporization, 

 and the elastic force is consequently increased in a much 

 more rapid ratio than it would be in a permanent gas by the 

 same change of temperature. Conversely, if the tempera- 

 ture be lowered, a part of the vapor is condensed, the den- 

 sity is diminished, and the elastic force reduced more 

 rapidly than in a permanent gas. The density of saturated 

 steam is about $ of that of atmospheric air, when they are 

 both under the same pressure and at the same temperature. 



It has been determined experimentally that whatever may 

 be the pressure at which steam is formed, the quantity of 

 fuel necessary to evaporate a given volume of water is 

 always the same ; also the relation between the temperature 

 and pressure of saturated steam has been determined experi- 

 mentally, and from this tables have been formed giving the 

 relation between the pressure and volume of steam raised 



