October 28, 1004.; 



SCIENCE. 



567 



be connected to an exhausted receiver. When 

 a tube of this liind is moistened, so that the 

 pores are iilled with water, and the tube is 

 protected from evaporation, it can be ex- 

 hausted to a pressure equal to the vapor pres- 

 sure of water, and, if connected to a two-liter 

 receiver, it will maintain that difference in 

 pressure for a day or more without sensible 

 loss. It is evident that under these circiim- 

 stances the curvature of the capillary water 

 surfaces on the outside of the tube must exceed 

 the curvature of the capillary surface on the 

 inside of the tube by an amount sufficient to 

 produce a pressure of nearly one atmosphere. 

 Suppose now that a tube of this kind be 

 buried in a moist soil having a moisture con- 

 tent sufficiently high to reduce the pressure 

 of the capillary water surface in the soil to 

 less than one atmosphere. Under these condi- 

 tions, a movement of moisture must take place 

 from the capillary spaces of the soil to the 

 capillary spaces in the porcelain tube where 

 a greater capillary pressure exists. Now the 

 curvature of the capillary water surface on 

 the outside of the porcelain tube must always 

 be sufficient to withstand the pressure of one 

 atmosphere produced by the exhaustion of the 

 tube, hence the water drawn into these spaces 

 must be forced through the tube by the dif- 

 ference in pressure between the outside and 

 inside of the tube, in order that the necessary 

 curvature of the outer capillary surface may 

 be maintained. A steady movement of water 

 into the tube will, therefore, take place. This 

 rate obviously depends : (1) Upon the differ- 

 ence in curvature of the capillary surfaces on 

 the outside of the tube and in the soil mass, 

 i. e., upon the moisture gradient; (2) upon the 

 resistance encoimtered by the water in mov- 

 . ing over the surfaces of the grains and through 

 the capillary spaces. 



The apparatus, with which the results given 

 below were obtained, consists of a Pasteur- 

 Chamberland filter tube, connected by means 

 of a short piece of lead tubing to an ex- 

 hausted two-liter bottle. Air-tight connec- 

 tions are readily made by the use of short 

 lengths of red rubber tubing, well coated 

 with thick cotton-seed oil. Preparatory to 

 placing the filter tube in the soil, a core of 



soil is removed by means of a tube, the ex- 

 ternal diameter of which is equal to 

 that of the smaller end of the porcelain tube. 

 The filter tube is slightly conical in form, so 

 that when it is forced into this hole, a good 

 capillary connection is established between the 

 walls of the tube and the soil. 



The apparatus was usually allowed to stand 

 for about twenty-four hours, when the ex- 

 hausted bottle was detached, and the water 

 which had been drawn into the apparatus re- 

 moved and measured. The porcelain tube was 

 not disturbed in removing the water, which 

 was drawn into a small flask by suction 

 through a fine tube extending to the bottom 

 of the porcelain tube. The apparatus was 

 then immediately put together again and ex- 

 hausted by means of an aspirator. 



The water thus removed not only represents 

 the amount of water which the soil has sup- 

 plied to the tube during the jjreceding period 

 of twenty-four hours, but it appears to be 

 identical in concentration* and composition 

 with the soil solution from which the plant 

 obtains its food.f Therefore, the determina- 

 tion of the amount and composition of its 

 soluble inaterial gives us at once the concen- 

 tration and composition of the soil solution. 

 The apparatus thus provides a simple means 

 of studying the changes which take place in 

 the solution from which plants obtain their 

 mineral food. 



In the following table, the second column 

 gives the rate in grams per hour at which 

 moisture was supplied by the soil to the filter 

 tube; the third, the electrical conductivity at 

 20° C. of the solution thus obtained, meas- 

 ured always in the same cell. These meas- 



* No measurable change is pi-odueed in the 

 concentration of a soil solution by filtering it 

 through a porcelain filter-tube which is free from 

 organic matter, and which has been thoroughly 

 washed. A tube clogged with chlorophyll, on 

 the other hand, does actually appear to filter oiit 

 a part of the solvent from some solutions, the 

 unfiltered portion immediately about the filter- 

 tube being more concentrated than the original 

 solution. See Bull. No. 19, Bureau of Soils, 

 U. S. Dept. of Agri., 1901. 



t The arguments in support of this statement 

 «ill be given in a subsequent paper. 



