AGENCY OF DEW. 97 



or quite enough from this source for the purposes of assimi- 

 lation. The dew conveys also carbonic acid and ammonia, 

 which we have seen are indispensable to vegetation. 



3. But aside from the direct agency of water as an aliment, 

 which we have already considered, the atmosphere performs 

 an important agency in yielding it, at the time when its pres- 

 ence is most needed to modify the effects which are produced 

 by the heat of the sun. This influence is always beneficial, 

 but almost indispensable in dry seasons. The moisture which 

 the air contains is conveyed, during the night, to all parts of 

 plants, not excepting their roots, if the soil is in a proper con- 

 dition to admit a circulation of air. In some countries, as 

 Egypt, there is no rain for several months ; this defect is, in 

 part, made up by the heavy dews which fall during the night, 

 and tend to restore the languishing energies of the vegetable 

 kingdom. It is owing to the direct agency of the dew upon 

 the roots of plants, that the earth should be stirred about 

 them, so as to keep it light, and always permeable to the air. 



4. The agency of water in the form of dew presents the 

 most striking illustration of its utility, and of its wise and 

 beautiful adaptation to the vegetable kingdom. All bodies 

 radiate heat into space during the night. The surface of the 

 earth gives off more heat than it receives, and the dew is de- 

 posited. But it should be remarked, that some bodies cool 

 much more rapidly than others ; hence these bodies will first 

 attract the particles of falling dew. Thus the grass plot is 

 wet, while the gravel walk is dry. The dew thus seems to 

 select the object which it would cherish, and, after having 

 ministered to the wants of every living plant, spends its su- 

 perfluity only on the naked earth or barren waste. 



5. The water thus distilled into the air, is precipitated, not 

 only in glistening dew drops, but in refreshing showers of 

 rain. This arrangement is exceedingly beautiful, if we con- 

 sider the fact, that were all the vapor condensed at once, it 

 would not cover the earth more than five inches in depth ; 



