130 Principles of Plant Culture. 



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 portion of the water escapes. If it is but half satu- 

 rated, it may be compressed somewhat without any es- 

 cape of the liquid, but if the compression passes a cer- 

 tain 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 

 temperature. 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 tempera- 

 ture 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 dew point depends, 

 therefore, upon the amount of water the air contains. 



