PHYSICAL (JUAllACTEliS OP THE .SOIL. 10.') 



from want of moisture; when, however, they occur afijfue 

 dust, they form too wet a soil, in which plants sufter from 

 the opposite cause." — [Tlamni's Lindiclrthschaft.) 



Every body has a definite power of condensing moist- 

 ure upon its surface or in its pores. Even glass, though 

 presenting to the eye a jjerfectly clean and dry surface, is 

 coated witli a film of moisture. If a piece of glass be 

 weighed on :i very delicate balance, and then be wiped 

 with a clean cloth, it will be found to weigh perceptibl}' 

 less than before. Exposed to the air for an hour or more, 

 it recovers the weight which it had lost by wiping; this 

 loss was water. (Stas. Magnus.) The surface of the 

 glass is thus proved to exert towards vapor of Avater an 

 adhesive attraction. 



Certain compounds familiar to the chemist attract water 

 with great avidity and to a lai'ge extent. Oil of vitriol, 

 phosphoric acid, and chloride of calcium, gain weight rap- 

 idly when exposed to moist air, or when placed contiguous 

 to other substances which are impregnated with moisture. 

 For this reason these compounds are employed for pur- 

 poses of drying. Air, for exaniple, is perfectly freed from 

 va})or of water by slowly traversing a tube containing 

 lumps of dried chloride of calcium, or phosphoric acid, or 

 by bubbling repeatedly througli oil of vitriol contained 

 in a suitable apparatus. 



Solid substances, which, like chloride of calcium, carbon- 

 ate of potash, etc., gather water from the air to such an 

 extent as to become liquid, are said to deliquesce or to be 

 dellques'ent. Certain compounds, such as urea, the char- 

 acteristic ingredient of human urine, deliquesce in moist 

 air and dry away again in a warm atmosphei'c. 



Allusion has been made in " How Crops Grow," p. 5.5, 

 to the hygroscopic water of vegetation, which furnishes 

 another striking illustration of the condensation of water 

 in porous bodies. 



The absorption of vapor of water by solid bodies is not 



