PRESSURE AND CHANGE OF STATE 67 



and thus the pressure is reduced and the boiling point is corre- 

 spondingly lowered. Under ordinary conditions the boiling 

 point is lowered 1C. for each 960 feet above sea level. The 

 change of solids to the liquid state is also affected by pressure. 

 The interior of the earth is believed to be hot enough to melt 

 all known rocks, and yet it cannot become liquid because of 

 the enormous pressure upon it. 



62. Compression of Gases. Solids and liquids differ as 

 regards the rate at which they expand when heated, but all 

 gases expand alike. For a rise of 1C. in temperature, they 

 increase one two hundred and seventy-third of their volume at 

 0C. Gases, being perfectly elastic, tend to expand indefinitely 

 if unconfined, and when subjected to pressure expand as soon 

 as the pressure is removed. When enclosed, the molecules are 

 always evenly distributed through the available space, no 

 matter what its size or shape. When another gas is introduced 

 into the same space, it also becomes evenly distributed through 

 it. Compressing the gas, however, develops pressure and the 

 higher the temperature of the gas the greater the pressure will 

 be, since the myriads of rapidly moving molecules constantly 

 beat upon the retaining walls. Liquids are practically incom- 

 pressible, but gases readily yield to pressure. If the tempera- 

 ture remains the same, doubling the pressure on a gas will 

 cause it to become half its original size, and trebling the pres- 

 sure will make it one-third its size, and so on. The physicist 

 explains this by the statement that "the volume of a gas varies 

 inversely as the pressure to which it is subjected." At a given 

 temperature and pressure a cubic centimeter of any gas will 

 have the same number of molecules in it. 



63. Heat and Compression. Not only will pressure hinder 

 a liquid from turning to a gas, but if sufficient pressure be 

 applied it may even be caused to turn back to the liquid 

 again. When pressure is applied to a gas, however, a certain 

 amount of heat develops and most gases cannot therefore be 



