164 HOW cuops fj:ed. 



only dependent on the natin-e of the substance and i'ibT 

 amount of surface, but is likewise influenced by external 

 conditions. 



The raj)idity 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 unaflected 

 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 which 

 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 tha 

 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. 

 " 66° " 11.9 " " " " 



" 77° " 10.:2 " " " " 



" 88° " 8.7 " " 



Knop calculates on the basis of his numerous observa- 

 tions that hair and wool, which aie more hygroscopic than 

 most vegetable and mineral substances, if allowed to ah 

 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. 

 " 55° " 15.5 " " 

 " 33° " 19.3 " " 



