FORMATION OF VAPOR, SATURATION. 109 



of 0, a pressure of 36 atmospheres* was required to produce conden- 

 sation. 



In 1877, M. Pictet succeeded in liquefying oxygen by subjecting it 

 to a pressure of 300 atmospheres ; at the close of the same year, M. 

 Cailletet effected the liquefaction of nitrogen, hydrogen, and atmos- 

 pheric air. Such experimental results point to the general conclusion 

 that all gases are the vapors of liquids of different kinds, f 



60. Formation of Yapor, Saturation. The major- 

 ity of liquids, when left to themselves in contact with the 

 atmosphere, gradually pass into the state of vapor and dis- 

 appear. This phenomenon occurs much more rapidly with 

 some liquids than with others. Thus, a drop of ether dis- 

 appears almost instantaneously ; alcohol also evaporates very 

 quickly ; but water evaporates much more slowly. If water 

 be introduced into a space containing dry air, vapor is im- 

 mediately formed ; if the temperature be increased, or the 

 space enlarged, the quantity of vapor will be increased ; but 

 if the temperature be lowered, or the space diminished, some 

 portion of the vapor will be condensed; in all cases the 

 pressure of the air will be increased by the pressure due to 

 the vapor thus formed. The formation of vapor is inde- 

 pendent of the presence of air or of its density, the only 

 effect which the air produces being a retardation of the 

 time in which the vapor is formed. If water be introduced 

 into a vacuum, it is instantaneously filled with vapor, but 

 the quantity of vapor is the same as if the space had been 

 originally filled with air. 



While the supply of water remains, as a source from 

 which vapor can be produced, any given space will be 

 always saturated with vapor, i. e., there will be as much 

 vapor as the temperature admits of. If the temperature be 

 lowered, a portion of the vapor will be immediately con- 

 densed, and become visible in the form of a liquid ; but if 



* An atmosphere denotes the pressure due to a column of mercury 29.9 inches in 

 height. 



t Begant's Hydrostatics, p. 136. 



