Osmosis. 43 







The diagram on the left may represent a raisin, dry 

 bean or other dry seed placed in water, the upper circle 

 showing the conditions just as placed in the water, and 

 the larger one after the water has diffused through the 

 wall, dissolving the substances on the inside, which pro- 

 duce the osmotic pressure. In this diagram it is supposed 

 that the surrounding membrane is porous enough to let 

 the molecules of water pass readily through by the or- 

 dinary laws of diffusion, but the molecules of the sub- 

 stance inside, which is dissolved by the entering water, 

 are too large to be able to pass out into the wa* ter by 

 diffusion, and the result is they simply strike against 

 the membrane, distending it in exactly the same way that 

 the molecules of air in a rubber ball or rubber bicycle 

 tire distend that. 



In the diagram on the right in Fig. 6, the reverse con- 

 ditions are represented. A green or fresh fruit is sup- 

 posed to be placed in a solution of sugar, whose molecules 

 are too large to readily pass into the fruit through its 

 wall, while the contained sap of the fruit readily diffuses 

 outward into the sweetened water. Under these con- 

 ditions the molecules of sugar, striking against the fruit 

 on all sides at once, develop so much pressure that the 

 juices of the berry are squeezed out of it as water might 

 be forced out of a sponge, and its volume is reduced from 

 the large size in the upper part of the diagram to the 

 smaller one in the lower portion. 



60. Measurement of Osmotic Pressure. It was a long 

 time after the discovery of osmotic pressure before satis- 

 factory means for measuring its full intensity were de- 

 vised. The parchment and animal or vegetable mem- 

 branes which were first used in such studies were either 

 not sufficiently impervious to the pressure producing mole- 

 cules or else their strength was not great enough to allow 

 the full measure of pressure to develop and the result was 

 the early experiments failed to show how powerful osmotic 

 pressure may become when the conditions are all favor- 

 able. 



