198 SOILS. 



faults, the more as the soils used are only designated in general 

 terms, as " garden soil," " loam," " peaty land," etc., without 

 any definite indication of their actual physical or chemical con- 

 stitution. The writer therefore undertook to correlate these 

 coefficients, determined with respect to completely saturated 

 air, with the physical composition of certain soils, as deter- 

 mined by means of the methods heretofore described. 



Some of the data so obtained are given in the table of physical soil 

 composition on page 93, chapt. 6. They have since been extensively 

 supplemented by additional determinations, but without materially 

 changing the coefficients approximately corresponding to the several 

 designations accepted in farm practice. Experiments conducted by 

 the writer have conclusively shown that Knop's law of decrease oi 

 absorption with rise of temperature not only is not true ior fully saturated 

 air, but must be reversed ; the fact being that the amount of water 

 absorbed by the soil increases in a fully saturated atmosphere (i.e., in 

 presence of excess of water) as the temperature rises, at least between 

 15 and 35 degrees Cent. Thus, fine sandy soil which at 15 absorbed 

 2% of moisture, took up 4% at 34 ; while loam soil absorbing 7 r / at 

 15, showed nearly g r / at 35 ; an increase of 2% in each case. But 

 in partially saturated air J it was found that, as stated by Knop, the 

 amounts absorbed steadily decrease, though not according to the law 

 announced by him. Taking as a unit the moisture absorbed at 15, 

 it was found that in air three-fourths saturated, -| of the unit was taken 

 up by the soil ; at half saturation, nearly the proportional amount ; but 

 at one-fourth saturation the earths absorb materially more than a similar 

 proportion, being then capable of withdrawing moisture from greatly 



layer exceeded about one millimeter, a long time was required for full saturation ; 

 during which inevitable changes of temperature would bring about a deposition of 

 dew on the soil, greatly exaggerating the absorptive coefficient. 



In the chamber used at the California station for soil saturation, dimensions 12 

 X 18 X 19 inches high, the same soil was exposed on a shelf close to the sur- 

 face of the water, another midway up, a third near the lower surface of the cover; 

 liquid water being in the bottom of the chamber, and the rest covered with wet 

 blotters. It was found that despite these precautions, the lowest soil layer 

 absorbed in the same time as much as ^ more than the uppermost one. 



2 The partial saturation to a definite extent was effected by means of solutions 

 of calcium chlorid of different degrees of concentration, according to the de- 

 terminations of Wullner (Pogg. Ann.). These solutions were placed in a wide, 

 flat dish, over which a layer of soil I mm. in thickness was exposed, all being 

 covered with a bell glass lined inside with the same solution, so as to insure equal 

 saturation. 



