60 THE WATER-SUPPLYING PO^\nER OF THE SOIL 



of a pipette graduated so as to be read to hundredths of a cubic centi- 

 meter. The pipette contained water, its meniscus covered by a layer 

 of oil to retard evaporation. The looped rubber tube allowed the 

 water level in the pipette to be kept approximately constant (with 

 reference to the water level in the horizontal jar) during a series of 

 observations, thus insuring a practically constant and low hydrostatic 

 pressure upon the membrane. 



The osmometer was filled, in the erect position, with freshly prepared 

 sugar solution, by means of a small glass tube reaching from its open 

 end nearly to the membrane. It was emptied, after an experiment, by 

 placing the bulb upward and then forcing air into it through the same 

 small tube. Between tests the osmometers were rinsed out and kept 

 in distilled water, the bulbs also filled with water. 



In order that different osmometers might be used and that their data 

 might be rendered comparable, it was necessary for each instrument to 

 be calibrated or standardized. The area of each membrane was accord- 

 ingl}" determined as accurately as possible, the membrane surface being 

 treated (with its slight bulge when supporting a few centimeters of 

 water column) as a portion of either a spherical or ellipsoid surface of 

 revolution (the openings of the thistle tubes were elliptical in some 

 cases) . From the actual area of the osmotic membrane was obtained, 

 in each case (by dividing by 10), a correction coefficient by which each 

 actual reading was to be multiplied in order to give approximately the 

 reading which should have been shown had the area in question been 

 just 10 sq. cm. All rates given below have been thus corrected, and 

 all refer to the hour as time period, so that each of these rates represents 

 the number of cubic centimeters of water passing in onehourtlirougha 

 surface with an area of 10 sq. cm. Every osmometer was tested several 

 times, operating against distilled water, and any considerable leakage 

 of sugar or unusual behavior of the rate of water intake thus detected 

 was considered sufficient reason for discarding. 



In both water tests and soil tests temperature readings (upon water 

 or soil) were made whenever the osmometers were read. For this 

 purpose a thermometer was inserted in the jar, alongside the osmometer, 

 its stem (and the portion of the scale used) extending out horizontally 

 through the cork stopper or cotton packing. It would have been 

 much more efficient and generally satisfactory if the temperature had 

 been maintained constant throughout each series, but a constant-tem- 

 perature room was not available for this work. As will appear later, 

 and as was to be expected, temperature seems to be a very important 

 variable in the soil complex that determines the possible rate of water 

 supply, and this feature should receive careful attention if this sort of 

 study is pushed forward. 



