PHYSICAL CHARACTERS OF THE SOIL. 163 
from want of moisture; when, however, they occur as fine 
dust, they form too wet a soil, in which plants suffer from 
the opposite cause.”—(Hamm’s Landuwirthschaft.) 
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 perfectly clean and dry surface, is 
coated with a film of moisture. Ifa piece of glass be 
weighed on avery delicate balance, and then be wiped 
with a clean cloth, it will be found to weigh perceptibly 
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 water an 
adhesive attraction. 
Certain compounds familiar to the chemist attract water 
with great avidity and toa large 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 example, is perfectly freed from 
vapor of water by lowly traversing a tube containing 
lumps of dried chloride of calcium, or phosphoric acid, or 
by bubbling repeatedly through oil of vitriol eontnined 
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 ax to become liquid, are said to deliquesce or to be 
deliquescent. Certain compounds, such as urea, the char- 
_ acteristic ingredient of human urine, deliquesce in moist 
air and dry away again in a warm atmosphere. 
Allusion has been made in “How Crops Grow,” p. 55, 
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 adild bodies is not 
