ABSORPTION 



227 



i. Physical Method. For the purpose of measuring osmotic pressure by 

 physical methods, it is customary to make use of an apparatus similar to that 

 represented in Fig. 91, which consists of an earthenware vessel (a), into the 

 upper open end of which a tall vertical glass tube has been hermetically sealed. 

 The pores of the earthenware vessel have been filled by a membrane made by 

 precipitating ferrocyanid of copper within them. This membrane is freely 

 permeable to water, but impermeable to certain substances in solution, e.g., 

 cane-sugar. Such a membrane, which permits the passage of the molecules^of 

 the solvent but not the molecules of the dissolved substance, is termed a semi- 

 permeable membrane, and its use is absolutely necessi- 

 tated when it is desired to obtain the actual pressure 

 exerted by any given substance in solution. An ap- 

 paratus of this character is termed an osmometer. 



When, therefore, the osmometer containing a solution 

 of cane-sugar is placed in the vessel (b) containing water, 

 the following phenomena occur, viz.: an ascent of the 

 cane-sugar solution in the vertical glass tube, and a de- 

 scent of the level of the water in the vessel b. These 

 phenomena continue until the level of the fluid in the 

 glass tube reaches a certain height, when it becomes sta- 

 tionary, and no further effect takes place. 



In explanation of the foregoing phenomena it may be 

 said that the molecules of the sugar strike or press against 

 the outer layer of the molecules of the solvent, which at 

 all points are in contact with the rigid walls of the earthen- 

 ware vessel, except at the open extremity of the vertical 

 glass tube. Inasmuch as the rigid walls of the osmometer 

 prevent any outward displacement of the molecules of the 

 water, the force of the impact of the sugar molecule is 

 directed against the molecules at the extremity of the 

 vertical tube which are in consequence pressed or 

 pushed upward a certain distance. Because of the loss 

 of energy due to the impact, the sugar molecule does not 

 rebound with the same velocity, and hence time is per- 

 mitted for the molecules of the water to pass into the 

 sugar solution, to occupy the space, and thus maintain 

 the level of the fluid in the vertical tube. (For the reason 

 that the osmometer is permeable to water, the molecules 

 will pass outward as well as inward though more will 

 pass in a unit of time in the latter, than in the former 

 direction, until equilibrium is established.) The pressure 

 of the sugar molecules continuing, the level of the fluid in 



Solution 



*>/ 

 Cane Sugar 



a 



the glass tube continues to rise and the level of the fluid in p IG , 91. AN OSMOMETER. 

 the vessel, b, continues to fall until the force of gravity 



prevents any further upward movement of the molecules of sugar against the 

 outer film of the molecules of the water. The difference in the level of the two 

 fluids expressed in millimeters of mercury is taken as a measure of, and equal 

 to, the pressure of the sugar in solution. A i per cent, solution of cane-sugar 

 at a temperature of from i3C. to i6C., as determined by this method, exerts 

 an osmotic -ps^ure of about 535 mm. Hg.; a 2 per cent, solution exerts an 

 osmotic pressureSpproximately twice this amount. 



Experiments made with this and similar osmometers show 



1. That the osmotic pressure of any substance in solution is proportional to the 



concentration, providing the temperature is constant. 



2. That when the concentration is constant the osmotic pressure ns< s with, a: 



id nrrmnrtinnal to. the temDCrature. 



