OSMOSIS AND IMBIBITION 231 



pressures in the same way that a gas exerts a pressure. In other 

 words, he came to the conclusion that osmosis was a molecular 

 phenomenon comparable to the pressure exerted by gases when 

 confined. The results obtained by Pfeffer, as shown above, can 

 be nicely interpreted in this manner. Doubling the concentration 

 doubles the number of molecules and hence doubles the pressure. 

 Increasing the temperature increases the pressure; and Van't 

 Hoff showed that, as in gases, the osmotic pressure is proportional 

 to the absolute temperature. He further showed that the osmotic 

 pressure which the majority of substances give when dissolved in 

 a liquid, is the same as that exerted by a gas at a given tempera- 

 ture with the same number of molecules in a given volume. To 

 illustrate: At normal temperature and normal pressure (one at- 

 mosphere) a gram molecule (the molecular weight in grams) of a 

 gas occupies a volume of 22.4 liters, so that, if the above law holds 

 in osmosis, a gram molecule of sugar dissolved in 22.4 liters of 

 water should give an osmotic pressure of one atmosphere. Sugar 

 has a molecular weight of 342. Hence 342 grams of sucrose dis- 

 solved in one liter of water should give 22.4 atmospheres of pres- 

 sure. A 1% solution would then give under these conditions a 

 pressure of about 0.69 atmosphere. The results found in various 

 ways by experiment show a 1% solution to give 0.62-0.71 atmos- 

 phere. That found by Pfeffer was 0.68 atmosphere, which is seen 

 to be in very close agreement with the calculations made on the 

 basis of the gas laws. This example is sufficient to show the justi- 

 fication of Van't Hoff 's view. 



Plasmolysis. — The effect of different substances on osmosis 

 was tested by de Vries, the famous Dutch botanist (to whom we 

 also owe the mutation theory of evolution), who used living plant 

 cells instead of artificial membranes to answer this question. It 

 will be remembered that the cell, on the outside nearest the cell 

 wall and on the inside nearest the vacuole, possesses a plasma mem- 

 brane which is differentially permeable, and it is these membranes 

 which are of prime importance in the osmotic phenomena of the 

 cell rather than the cell wall, which is only slightly differentially 

 permeable. When a cell is placed in a solution of pure water, the 

 water enters and causes the cell to distend. The protoplast is 

 pushed firmly against the cell wall, and the cell in this water- 

 gorged condition is said to be turgid. 



If, on the other hand, the cell is placed in a strong salt or sugar 



