Injury from Cold During Growing Period. 127 



will be set free or precipitated. If the air were only half 

 saturated at 50° F., its temperature could be reduced 

 considerably before any of its water would be precipitated; 

 but when a certain degree of cooling is leaehed, the air 

 will no longer be able to hold all of its water, and a part 

 will be precipitated. The cooling of the air corresponds 

 to the compression of the sponge. The atmosphere 

 always contains more or less water in the form of watery 

 vapor, and the temperature at which any portion of the 

 atmosphere on cooling begins to precipitate a part of its 

 water, is called the dew point. The temperature of the 

 devi' point depends, therefore, upon the amount of water 

 the air contains. When the dew point is above the 

 freezing point of water (32° F., 0°C. ), the precipitation 

 is in the form of dew or rain; but when it is below the 

 freezing point of water, the precipitation is in the form 

 of hoarfrost or snow. One more principle needs to be 

 explained, and we are ready to understand 



207. How Frost may be Foretold. We know that 

 sprinkling the lloor of a room cools the air, even though 

 the water used is no cooler than the air of the room. 

 This is because the air in taking up watery vapor ab- 

 sorbs heat, but this heat is set free again when the watery 

 vapor is precipitated. A steam radiator gives out heat 

 because the steam within it is condensing into water. It 

 follows that when the dew point of the atmosphere is 

 reached, a very considerable amount of latent heat is 

 given off, which checks the fall of temperature. The 

 temperature of still air, therefore, rarely falls much below 

 the dew point, and since the latter at any given tempera- 

 ture depends upon the amount of moisture in the air, if 



