258 EVAPORATION. 



fully as when its operations first began. Let us then 

 examine this operation of nature, so important in its 

 effects, so interesting in regard to the phenomena by 

 which it is attended. 



We will first inquire into the cause of evaporation. 

 Heat appears to be the great agent, by which the process 

 is carried on. Perhaps the air itself may have some 

 tendency to raise water ; but various experiments prove 

 the great influence of heat in the formation of vapor. 

 An illustration of this may be seen in the rapidity with 

 which the ground dries after a rain in summer, compared 

 with the slowness of the same process in autumn or win- 

 ter. When the temperature of water is raised above 212 

 degrees, it assumes the form of vapor so rapidly 

 as to cause that bubbling which we term boiling. This 

 process differs in some respects from the formation of 

 vapor at the common temperature of the atmosphere. 

 The distinction between them is sometimes expressed by 

 the terms vaporization and evaporation. There is how- 

 ever so much connexion between them, that we may with 

 propriety examine them together. Water may be made 

 to pass off entirely in vapor, though kept at a temperature 

 far below 212 degrees. Moisture collects on the cover 

 of a kettle, when placed over the fire, long before the 

 liquid has reached the boiling point. But it is not till it 

 has reached that point, that the expansive power of heat 

 is sufficient to enable the water to assume the form of 

 vapor, while pressed by the superincumbent liquid. At 

 that point it becomes sufficient, and the water near the 

 bottom expanding into vapor, and rising rapidly to the 

 surface, causes the appearance, which is termed boiling. 

 On high mountains water boils at a much lower tempera- 

 ture than in valleys. The pressure of the air on the 

 surface of the water is less, and consequently the resist- 

 ance to the formation of vapor is diminished. On the 

 top of Mont Blanc, the highest of the Alps, water boils 

 at 187 degrees. In the exhausted receiver of an air 

 pump, it boils at a still lower temperature. Ether is so 

 volatile that it boils at almost any temperature, when the 

 pressure of the atmosphere is removed. If a watch- 

 crystal containing a little ether, be placed in another 



