148 



PHYSIOLOGY OF NUTRITION 



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 c 



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dure has been recently developed at 

 the Laboratory of Plant Physiology of 

 the Johns Hopkins University, adequate 

 to demonstrate actual water tensions 

 at least approaching one-and-one-half 

 atmospheres. 



This procedure meets the first of the 

 above-mentioned difficulties by using, as 

 evaporation cell, a previously tested cyl- 

 inder of porous porcelain, closed and 

 rounded at one end. The cylinder is 

 attached by means of a tightly fitting 

 rubber stopper to the end of a long, 

 vertically placed, thick-walled glass tube 

 of small bore (barometer tubing), bent in 

 the form of an inverted J (see Figure 84a). 

 The second difficulty is overcome partly 

 by an initial cleansing of the parts with 

 suitable reagents (hot chromic acid for the 

 glass, and boiling sodium hydroxide solu- 

 tion for the porcelain and rubber portions) 

 and partly (after the apparatus has been 

 assembled) by causing a stream of boiling 

 distilled water to filter through the system 

 under suction for about an hour. The hot 

 water enters through the walls of the 

 temporarily submerged porous cylinder, 

 and flows out at the lower end of the 

 vertical tube, which is connected to a 

 suction pump by means of a suitable bottle 

 fitted with a two-hole rubber stopper. 

 This bottle contains enough mercury to 

 submerge the lower end of the vertical 

 tube, and thus serves as both suction 

 flask and mercury reservoir. The wetting 

 process seems to be hastened by rapidly 

 and repeatedly releasing and renewing 

 the suction for periods of a few minutes 

 at convenient intervals during the treat- 



Fig. 840. — Diagram of arrangement for demonstrating liquid tension by the general 

 method of Askenasy. (By Grace Lubin.) C, porous porcelain cylinder from which evaporation 

 is to proceed, temporarily submerged in water held in beaker B for preliminary suction treat- 

 ment. 5, rubber stopper; abed, thick-walled, small-bore glass tube; s, rubber stopper; R, 

 mercury reservoir; H, mercury; e, suction outlet; G, gas burner, (or electric hot-plate) for 

 heating water in beaker. After preliminary treatment beaker and burner are removed and 

 mercury rises in the tube. The necessary supports are not shown. 



