116 



EXPERIMENTAL GENERAL SCIENCE 



tie-tube half full of molasses, tying a piece of parchment paper 

 or other membrane over the open end, and inverting it in a 

 jar of clear water. The water at once begins to flow into the 

 thistle-tube through the membrane, and often increases the 

 bulk of the molasses so much that it will rise in the stem of the 

 thistle-tube 18 or 20 feet against gravity. As the molasses 

 slowly diffuses out into the water, the density of the two liquids 

 gradually becomes the same and the 

 column of liquid falls. Gases separated 

 by a membrane behave in the same 

 way, but the phenomena are most 

 noticeable in liquids. To certain kinds 

 of liquids and gases, membranes may 

 be semi-permeable; that is, they may 

 allow fluids to pass but retain sub- 

 stances dissolved in them. The cell 

 walls of plants with their lining of pro- 

 toplasm act in this way, and while per- 

 mitting the inflow of water and food 

 ^ materials, refuse to allow the matter 

 FIG. 38. Method of within the cells to escape. 



setting up osmosis experi- 



ment - Practical Exercises 



1. Fasten two strips of glass together in narrow V-shape using a 

 small piece of wood to keep the V open. Stand the strips on edge in 

 a dish of water and explain the behavior of the liquid. 



2. Account for the fact that if you touch one corner of a lump of 

 sugar to a liquid, such as coffee, the liquid will spread through the 

 entire lump. 



3. What use is made of capillarity in the wicks of oil lamps? 



4. Why is writing paper "sized" while blotting paper is not? 



