164 HOW CROPS FEED. 
only dependent on the nature of the substance and its 
amount of surface, but is likewise influenced by externa) 
conditions. 
The rapidity of absorption depends upon the amount 
of vapor present or accessible, and is greatest in moist 
air. 
The amount of absorption is determined solely by tem- 
perature, as Knop has recently shown, and is unaffected 
by the relative abundance of vapor: i. e., at a given tem- 
perature a dry soil will absorb the same amount of moist- 
ure from the air, no matter whether the latter be slightly 
or heavily impregnated with vapor, but will do this the 
more speedily the more moist the surrounding atmosphere 
happens to be. 
In virtue of this hygroscopic character, the soil whieh 
becomes dry superficially during a hot day gathers water 
from the atmosphere in the cooler night time, even when 
no rain or dew is deposited upon it. 
In illustration of the influence of temperature on th2 
quantity of water absorbed, as vapor, by the soil, we give 
Knop’s observations on a sandy soil from Moeckern, Sax- 
ony: 
1,000 parts of this soil absorbed 
At 55° F. 13 parts of hygroscopic water. 
oe 66° “cc 11.9 74 ce ce a4 
<4 if fa oe 10.2 oe “ 74 ce 
ce 88° ce 8.7 74 cc 74 6c 
Knop calcul:utes on the basis of his numerous observa- 
tions that hair and wool, which are more hygroscopic than 
most vegetable and mineral substances, if allowed to ab 
sorb what moisture they are capable of taking up, contain 
the following quantities of water, per cent, at the temper- 
atures named : 
At 87° Fah., 7.7 per cent. 
6c 55° 6c 15.5 6c 6 
“cc 32° ce 19.3 ‘T4 “ce 
