AGRICULTURAL TEXT-BOOK. 1 7 



the fall in temperature of the bodies on which it is deposited ; thence 

 the phenomenon cannot be attributed to anything more than a s mple 

 condensation of the watery vapor contained in the air, comparable in all 

 respects to that which takes place upon the surface of a vessel contain 

 ing a fluid that is colder than the air. The quantity of the moisture 

 dissolved in the atmoaphere is so much the greater as the temperature 

 is higher. In very warm climates the dew is so copious as to assist 

 vegetation essentially, supplying the place of rain during a great part of 

 the year. When the sky is clear and calm during the night, vegetables 

 cool down and very soon show a temperature inferior to that of the air 

 which surrounds them. Thus plants are often destroyed by frost in 

 spring, when the thermometer, a few feet above the ground, stands 

 above the freezing point. But clouds, preventing the rapid radiation, 

 also prevent plants cooling to the same point ; and on cloudy nights, 

 as also on those preceeding severe rain storm?, there is no dew. 

 The Farmers of Peru, South America, whose crops are often destroyed 

 by this nocturnal radiation, have long boon in the habit of making arti 

 ficial clouds by setting fire to a heap of wet straw or dung, and b} this 

 means raising a cloud of smoke which destroys the transparency of the 

 atmosphere, from which they have so much to apprehend. Unless the 

 plants and surface of the earth cool below the temperature of the air there 

 cannot be dew. (Boussingautt. ) 



55. But it is from springs, and the water otherwise contained 

 in the earth that plants cheifly receive their inorganic nourish 

 ment. As such water is always rising to the surface, it brings 

 with it whatever substances are held in suspension by it, and 

 deposits them either near the roots of the plants or on the sur 

 face, to be washed down again by rain. Thus the great marl 

 beds, which form the bottom of most of the small lakes of the 

 interior of Michigan, are deposits of lime dissolved in springs, 

 and which becomes a carbonate of lime, and therefore insoluble, 

 when exposed to the atmosphere. The great beds of nitrate of 

 potash and soda, (saltpetre,) and other salts, found on the sur 

 face in various parts of the world, are believed to owe their 

 existence to the same cause. Tables of the analyses of various 

 waters will be here given, by which it will be seen how impor 

 tant an element it is in the production of vegetation : 

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