CHAPTER IV. 

 PROPERTIES OF STEAM 



If we heat water in an open vessel, it soon reaches 

 the temperature of 212 degrees. If we continue to 

 heat it, it boils, or the energy imparted to its mol- 

 ecules is such that their vibrations become violent 

 enough to cause them to fly off and thus the vapor or 

 steam arises. If we now close the top of the vessel, 

 the molecules in their attempt to escape encounter 

 resistance and hence have to expend some of their 

 energy in overcoming this, that is, an extra quantity of 

 heat must now be given to the water to make it turn 

 to steam. If it rests under the weight of the atmos- 

 phere alone, or about 14 Ibs. to the square inch, the 

 water will boil at 212 degrees. If the pressure be 

 increased to 32 Ibs., the water will not boil until it 

 reaches a temperature of 254 degrees. If the pres- 

 sure be diminished to about six Ibs. to the square inch, 

 the boiling point is about 170 degrees, hence the law 

 or general statement: An increase of pressure on 

 the surface of a liquid raises the temperature at which 

 it boils; a decrease of pressure lowers the tempera- 

 ture. The temperature of boiling water always cor- 

 responds to its pressure. We call steam which is in 

 contact with the surface of water, saturated steam. 

 Under the law just given, its temperature depends on 

 the pressure in the boiler. 



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