UNFAVORABLE TEMPERATURE 121 



It is this extreme that we have chiefly to fear and to guard 

 against during the growing period. 



205. The cause of hoarfrost. — A sponge saturated 

 with water cannot be compressed in the least unless a 

 part of the water escapes. If it is but half saturated, it 

 may be compressed somewhat without any escape of the 

 liquid, but if the compression passes a certain limit, the 

 water will begin to escape. 



The air is like a sponge in being capable of taking up 

 a certain amount of water. But the amount of water 

 the air can take up depends much upon its temperature, 

 its capacity for water increasing as the temperature rises, 

 and decreasing as it falls. 



Suppose a given amount of air at a temperature of 

 50° F. has taken up all the water it can hold at that tem- 

 perature. It is clear from what has just been said that if 

 the temperature of this air is reduced, some of its water 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 reached, 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 the atmosphere on cooling 

 begins to precipitate a part of its water is called the dew 

 point. The temperature of the dew point depends, there- 

 fore, upon the amount of water the air contained. 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. 



