100 



GENERAL SCIENCE 



to pass rapidly through a porous partition. Where liquids 

 or gases upon opposite sides of a partition are of different 

 densities, the more rapid inflow of the less dense fluid causes 

 a pressure within. Though the rise of water through the 

 stems of plants, even to the very tops of the giants of the 



redwood groves of the Pacific 

 Coast, may not be wholly un- 

 derstood, this osmotic pressure is 

 in part an explanation of it. The 

 height of the liquid column in 

 an experiment with a diffusion 

 bulb measures the large pressure 

 developed there. Plants at times 

 do not have a sufficient supply 

 of water in hot summer days to 

 make good their loss through the 

 leaves. If the inflow of sap into 

 the cells of a plant is insufficient 

 to keep them filled, the plant wilts 

 until such time as these conditions 

 change. The term osmosis implies 

 interchange of like fluid states 

 through a porous partition, as 

 liquid mixing with liquid or gas 

 mixing with gas. However, some 

 liquids osmose so very slowly that 



their passage through the membrane becomes a negligible 

 quantity, and the flow becomes practically a mass movement 

 of one of the fluids in one direction only. 



FIG. 35. Osmosis. The more 

 rapid inflow of liquid from A 

 to B has elevated the liquid 

 surface within from b to a. 



SUMMARY 



Where like states of matter intermingle by reason of their molecular 

 movements alone, as in the mixing of two free gases, the change is 

 known as diffusion. Where the states of matter are unlike, as ink into 



