262 EVAPORATION. 



occasions no dampness. The quantity which it is capa- 

 ble of so containing, varies with the warmth and density 

 of the air. It decreases ' from below upwards, and from 

 from the equator to the poles. The air has been com- 

 pared to a sponge, expanding and becoming capable of 

 containing more water, by heat, and contracting by cold.' 

 Air at the freezing point can hold in solution T -^ of its 

 own weight in water : at 59, ^ at 86, V> anf l so on, 

 its power doubling at every increase of temperature equal 

 to 27 degrees. In a hot summer day, the air holds a 

 great quantity of water in solution ; yet we are wholly 

 unconscious of its presence unless it is by some means 

 deprived of its heat, and thus rendered visible. If, in a 

 warm day, a tumbler be filled with cold water, the outside 

 of it will almost immediately be covered with dew. The 

 tumbler being colder than the surrounding air, takes away 

 a part of its heat, and the moisture which that heat kept 

 in solution is rendered visible on the outside of the tum- 

 bler. When the dew thus deposited is very abundant, it is 

 justly regarded as a sign of rain. Other things being 

 equal, the quantity deposited will be in proportion to the 

 amount of water then held in solution by the air. If this 

 amount is great, a small change in the state of the air 

 may produce rain. If it is small, a greater change will 

 be necessary, and consequently the probability of rain is 

 less. But vapor sometimes rises in so great quantities 

 that the air cannot dissolve it all. Sometimes by a 

 change of temperature, or some other means, the air loses 

 a part of its power to contain water in an invisible state. 

 A part of the water will then become visible in the form 

 of a cloud. 



Clouds, then, are collections of vapor in the air, render- 

 ed visible by condensation. They seldom rise very high. 

 Sometimes they rest upon the earth's surface, constituting 

 what is termed fog. Sometimes they are a mile above 

 the surface of the earth, sometimes more ; but they sel- 

 dom rise higher than two or three miles. Very thin 

 fleecy clouds, however, sometimes rise to the height of 

 4 or 5 miles. But why do they not rise to the surface of 



