AS INDICATED BY OSMOMETERS. 61 



ARRANGEMENT OF OSMOMETERS TO OPERATE AGAINST 



THE SOIL. 



Since the work in hand was merely of a preliminary character, the 

 aim being primarily to find out whether such methods as these may 

 be regarded as promising, no attempt was made to study any but 

 artificial soil mixtures, and the packing of these soils was always really 

 uncontrolled. The adaptation of the collodion or other similar mem- 

 brane to actual use in the field and among the roots of growing plants 

 has yet to be accomplished, and may well make several studies for 

 students who sufficiently realize the importance of this general problem 

 to undertake this work. The method, as we have employed it, was 

 devised only for testing the availability of the collodion membrane and 

 is not at all suited to actual use in the field. This point is emphasized 

 to warn the reader against the supposition that we intend to offer here 

 anything approaching a practical arrangement for field work. 



Such being the case, the application of our membranes to the soil 

 were made in an exceedingly crude manner. A sample of the soil to 

 be tested was more or less closely packed into the basal half of a jar 

 (about 8 cm. in diameter and 12 cm. high), such as was employed for 

 the water tests, the free soil surface was rendered as smooth and uni- 

 form as possible, and the osmometer membrane was gently appressed 

 against this surface. The jar and osmometer tube were again hori- 

 zontal, as in the water tests, the soil surface to be tested and the 

 membrane in contact therewith being approximately vertical. Evapo- 

 ration from the soil around the instrument was nearly prevented, and 

 rendered negligible in amount, by filling the remaining portion of the 

 jar with dry cotton, pressed somewhat firmly into place. 



It was soon found that the osmometer membrane failed to maintain 

 adequate contact with the soil, which frequently tended to shrink 

 away from the film. To obviate this difficulty an arrangement of a 

 lever, cord, and suspended weight was added, the pressure of the 

 weight being transmitted through cord and lever so as to tend con- 

 tinuously to press the osmometer against the soil. Since the thistle 

 tube could slide rather freely through the cotton packing, any shrinkage 

 of the soil or other serious disturbance of the contact between mem- 

 brane and soil surface was thus automatically and satisfactorily ad- 

 justed. Figure 2 presents the arrangement as actually used. It will 

 be noted that the essential members of the osmometer are disposed 

 quite as in the water tests. 



In both water and soil tests, readings on the graduated tube were 

 obtained at 15-minute intervals, and these, after correction to give the 

 rate for the standard area of 10 sq. cm., have been reduced to hourly 

 rates cubic centimeters in all cases. 



The soil employed was a mixture of three-fourths sand and one- 

 fourth heavy loam, the same mixture as one of those employed by 



