VAPOR. 71 



no longer retain. This mist would gradually deposit 

 and collect in the form of water, and if measured, 

 would be found to make more than half a cubic inch. 

 Something less than half a cubic inch would remain 

 as invisible vapor in the cooled air. If the air were 

 cooled further, part of this would be condensed to 

 water. 



What is 163. UNSATURATED AIR. Air that does 



said of unsat- not con tain its complement of water will 



urated air and 



its moisture ? not yield any by slight cooling. It would 

 be like slightly compressing a half-filled sponge. But 

 as the cooling proceeds, the vapor becomes so dense 

 that further cooling will cause a deposition of moisture. 

 A cubic yard of air at 75, containing only half a cubic 

 inch of water in the form of vapor, would yield none 

 on being cooled down to 50. At this point the formation 

 would commence. If it contained originally less than 

 half a cubic inch, it would have to be cooled still lower 

 before any moisture made its appearance. The less 

 the moisture, in short, the more cold it would require 

 to wring it out. 



Is the quantity 164. QUANTITY OF VAPOR IN THE AT- 



MOSPHERE As has been already stated, 



ways propor- the capacity of air for vapor is in propor- 



tioned to its . m , r . - 



warmth? tion to its warmth. The air of summer 

 can therefore contain more than that of winter ; and 

 it frequently does so. But this is not necessarily the 

 case, for the capacity for moisture is not always filled. 

 Hot air over a desert, for example, contains less mois- 

 ture than cold air over the sea. And in the same lo- 

 cality, and during the same season, the quantity of 



