THE ABSORPTION OF FOODS 



116 



water around it may represent the blood. Under these con- 

 ditions there would seem to be no special reason why there 

 should be a flow of liquid in either direction; but nevertheless, 

 as a matter of fact, the contents of the tube soon begin to 

 flow out through the wall of the tube into the jar and at the 

 same time water flows into the tube. They flow at different 

 rates of speed, however, in the two directions, the water, in 

 this particular case, flowing into the 

 tube much more rapidly than the 

 sugar solution flows out. This proc- 

 ess is called osmosis. 



Since the membrane contains no 

 holes large enough to be seen with a 

 microscope, the condition in which 

 water or any other liquid could pass 

 through it must necessarily be that of 

 a very fine state of division, doubt- 

 less in what chemists call molecules. 

 Molecules are the smallest forms 

 which any substance can take and 

 yet maintain its own characteristics 

 and are, of course, invisible. It is 



known, however, that molecules are of different sizes, a 

 sugar molecule containing practically eight times as many 

 parts (atoms) as a water molecule. If, then, sugar molecules 

 and water molecules are mingled inside the tube, and only 

 water molecules are on the outside, many more water mole- 

 cules are in contact with the membrane on the pure water side 

 than on the other; for each sugar molecule takes up much of 

 the space on the inside of the tube. 



Physicists tell us that all molecules are in very rapid motion, 

 though each, in a general way, remains in its own "play- 

 ground." These molecules of water and sugar then are con- 

 stantly hitting against the walls of the membranous tube, and 

 some will pass through the mfinitesimally small pores in it. 



FIG. 63. DIAGRAM IL- 

 LUSTRATING OSMOSIS 



