WATER, AND ITS USES 47 



enough to cover the bulb, and let it hang suspended there : till next 

 day when the changes that follow will come up for discussion. 



1. Describe the change observed. What is the explana- 

 tion of it? When liquids mix (intermingle) by passing 

 through porous partitions by reason of their molecular 

 motions alone, the phenomenon is known, as osmosis. This 

 term covers the mixing of gases in like manner. What 

 must be true of the freedom of molecules to move about, 

 that osmosis, whether of liquids or of gases, can occur? 

 What is done to give solids the necessary molecular freedom 

 for osmosis? What evidence has there been of a greater 

 flow in one direction than in the other? What simple 

 explanation for this may be offered ? As the liquid column 

 mounts higher and higher, what sustains it in the tube? 

 What measures the value of this osmotic pressure f 



2. Some substances in solution (like white of egg, and 

 solutions of starch and of glue) osmose but little if at all. 

 Such substances have been called colloids, distinguishing 

 them from substances known as crystalloids whose solutions 

 osmose readily. 



3. What condition of tissues is necessary that osmosis 

 of gases may take place in the lungs whereby oxygen 

 gets into the blood and carbon dioxide gets out? Dis- 

 cuss in detail this process of interchange of gases in the 

 lungs. 



4. W 7 hat is the great purpose of the various steps in the 

 digestion of food ? What is the chief end to be attained in 

 cooking food ? W T hat makes up the large bulk of the diges- 



1 A rubber stopper previously slipped on over the upper end of the glass 

 tube makes this suspension easily arranged. 



