28 AMERICAN AGRICULTURE. 



This immediately brings the surface to the dew point and 

 secures a copious deposit of moisture, which a friable soil 

 speedily conveys to every part of the roots. 



The dew point is attained "when the surface of any ob- 

 ject is below the temperature of the surrounding air. The 

 careful observer will not fail to discover the formation of 

 dew, not only for some time after the sun has risen, and 

 long before he sinks below the horizon, when the condition 

 above indicated exists ; but sometimes even in the fervor of 

 a mid-day sun, when the thick corn, or any luxuriant vege- 

 table growth repels his fierce rays from the earth. In many 

 instances, the rank, dark growing crops themselves, when 

 shielded from the sun's rays by their overspreading tops, 

 become rapid condensers of atmospheric vapor, and the plant 

 drinks in the wholesome and nutritious aliment at every 

 pore, and frequently collects a surplus, which streams down 

 its sides to the thirsty soil beneath. The principle is further 

 illustrated, by the deposit of moisture in large globules on 

 the surface of any object in the shade, which is sensibly be- 

 low the surrounding temperature ; as is shown by an earthen 

 or metallic vessel filled with cold water, and set in a warm 

 room on a summer's day. 



The proper capacity of soils for imbibing and parting 

 with moisture, gives them another decided advantage over 

 others, which have it in an imperfect degree ; as it is found 

 by recent experiments, that rich, porous soils, which are 

 readily penetrated by water and air, absorb the nutritious 

 gases, (oxygen, nitrogen, and their compounds, nitric and 

 carbonic acid, ammonia, &c.) largely from the atmosphere ; 

 and that they do this to an appreciable extent, only while 

 moist. The effect of this will readily be estimated, fro^ the 

 well-known, beneficial influence exerted on the growing 

 plant, by the presence of these important elements. 



Light colored clays, marls, and sands, are neither in their 

 mechanical texture, friableness, or color, the best suited to 

 promote the growth of plants. Peat soils, from a deficiency 

 of inorganic materials, and their too great affinity for water 

 in their natural condition, are even less adapted to the object 

 than either of the preceding. 



Schubler has found, that during 12 hours in the night 

 when the air was moist, 1,000 Ibs. of entirely dry quartz,* 



* Quartz, as analyzed by Bergman, gave 93 per cent, of silex ; 6 of 

 alumina ; and I of oxide of iron. It comes so near a pure silica, that 

 in treating of it agriculturally, we speak of it as silex or silica. 



