116 



ELEMENTS OF GENERAL SCIENCE 



Increased pressure raises the boiling point very decidedly. 

 In the steam boiler, where steam is not allowed to escape 

 until it has reached a high pressure, the pressure of the steam 

 has the same effect upon the boiling point as does the in- 

 creased pressure of the atmosphere. The boiling point under 

 additional pressure of 30 pounds to the square inch is!20.6C., 

 at 100 pounds it is 155 C., and at 300 pounds it is 180 C. 



FIG. 59. Boiling point upon a mountain 



Mount Rainier, Washington. The figures show boiling points (Fahrenheit scale) 

 at different elevations when the air pressure is thirty inches at sea level 



133. Other facts about steam. When water changes into 

 steam, it increases enormously in volume if it is free to 

 expand. Steam at atmospheric pressure may occupy about 

 1600 times as much space as the liquid water from which it 

 was formed. Since steam occupies so much more space than 

 water, a similar volume of it is correspondingly lighter. It 

 is in fact lighter than a similar volume of air. If water is 

 boiled in a closed vessel (as in a steam boiler) the steam 

 cannot expand; but as the temperature increases, the pres- 

 sure also increases, and finally the steam finds some way of 



