56 THE WATER-SUPPLYING PO^VER OF THE SOIL 



allowed to extend outward as far as the walls of the containing vessel, a 

 result readily attained by limiting the amount of the solution originally 

 poured on the mercury surface. After evaporation of the solvent has 

 progressed to a certain degree the marginal portion of the hardening 

 superficial film rises in an irregular manner so as to exhibit a scalloped 

 appearance. When this occurs it is time to lift the film, which is done 

 by inserting the fingers below it (in the mercury), seizing it by its mar- 

 gin, and stripping the whole semi-hardened membrane from the still 

 liquid solution lying below. It has proved advantageous inmiediately 

 to draw the lower surface of the membrane over a tightly stretched 

 wire, thus removing most of the liquid which adheres and which other- 

 wise frequently collects locally to produce thickened regions in the 

 finished product. 



The ''green" membrane thus obtained must be expeditiously 

 attached to a suitable support and it must then be matured. In our 

 work, the film of collodion, still wet with liquid solution on its lower 

 surface, was quickly applied to the slightly enlarged rim of the larger 

 opening of a glass thistle tube, the undried surface being against the 

 glass. The membrane was appressed firmly to the edge of the rim and 

 the outer portion was drawn backward and inward and ligatured 

 against the exterior surface of the flaring portion of the tube by means 

 of several turns of linen thread. At the time of attachment the mem- 

 brane was loose and its central portion sagged considerably into the 

 tube. If this precaution is not observed ruptures are apt to occur with 

 the later removal of the solvent. Drying was then allowed to occur, 

 by simple exposure to the air, until the membrane tightened to a 

 plane surface across the opening and resounded, in tambourine fashion, 

 on tapping with the finger-nail. Having attained this stage of ripening, 

 the membrane was placed in 85 per cent alcohol (85 volumes of alcohol 

 and 15 volumes of water), the alcohol solution bathing its external 

 surface only, and was thus allowed to remain for about 12 hours. 

 This procedure was found markedly to decrease its permeability to 

 cane-sugar, though it also lessened that to water. If this alcohol 

 treatment were omitted in our tests the resulting membranes were 

 sensibly permeable to cane-sugar — too much so for the work in hand. 

 After the alcohol treatment, which appears to diminish the degree of 

 dispersion in the coagulating water-collodion emulsion, the membrane, 

 on its mounting, was transferred to water. After a few hours in this 

 liquid the osmometer was ready for use. When not employed the 

 membrane was always kept in water. 



Each osmometer was first tested by filling bulb and tube with water 

 and suspending for an hour, with the membrane below, to determine 

 whether any molar movement of water ensued. If liquid water 

 appeared on the exterior surface the instrument was discarded. 



