SOILS. 25 



deposit of moisture, which a friable soil speedily conveys to 

 every part of the roots. 



Tlie dew point is attained when the surface of any object 

 is below the temperature of the surrounding air ; and the 

 careful observer will not fail to discover the formation of 

 dew, not only after the sun has risen, and long before he 

 sinks below (he 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 vegetable 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 at 

 every pore, the wholesome and nutritious aliment, and fre- 

 quently 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 vessel or object in the shade, which is sensibly below 

 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 penetra- 

 ted 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, from the well known bene- 

 ficial 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 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, 1000 Ibs of entirely dry quartz* or 

 common sand, did not gain a pound ; calcareous sand gained 

 2 Ibs ; loamy soil 21 Ibs ; clay loam 25 Ibs ; such as were 



* Quartz as analyzed by Bergman, gave 93 per cent, of silei; (5 of alumina ; and 

 1 of oxide of iron. It comes so near a pure silica, that in treating of it agricultu- 

 rally, we speak of it as silex or silica. 



A. 



