1024 



ABSORPTION 



which is known as the osmotic pressure, is responsible for the passage 

 ("pulling") of the molecules of water through the pores of the mem- 

 brane. In general, it may be said that the osmotic pressure of a solu- 

 tion is proportional to its molecular concentration, i.e., to the number 

 of molecules of the dissolved substance in a given volume of the solu- 

 tion. This fact implies that it differs with the character of the solu- 

 tions employed. Its force, however, is considerable at all times. 

 Thus, it has been determined that a 1.0 per cent, solution of cane- 

 sugar at 0C. exerts a pressure of 493 mm. Hg. Regarding its origin 

 little is known, but it is commonly believed that it is due to the 



FIG. 520. DIALYSER, CONSISTING OF A TUBE OF PARCHMENT PAPER IMMERSED INA VES- 

 SEL THROUGH WHICH A CONSTANT STREAM OF STERILE DISTILLED WATER CAN BE PASSED. 

 ( Wrobleski.) 



kinetic energy of the moving molecules. The greater their attraction, 

 the greater this pressure. 



While such simple arrangements as have just been described, 

 actually exist in our body, the most common interchanges take place 

 between crystalloids and colloids. The process of transferring these 

 substances through an animal membrane interposed between the 

 solution containing them and the water, \s known as dialysis. In 

 this case, the crystalloids traverse the membrane and enter the water, 

 while the colloids do not. But since the membranes in our body are 

 only approximately semi-permeable, they allow water to go through 



