OSMOSIS AND IMBIBITION 233 



Hydrostatic Pressure. — Instead of measuring the osmotic 

 pressure directly, we measure in the case cited above the hydrostatic 

 pressure which is developed by the osmotic pressure. A sugar 

 solution in an open beaker possesses osmotic pressure, but this 

 is not measured by the methods thus far described until this os- 

 motic pressure is changed into hydrostatic pressure by separating 

 it from pure water by a semipermeable membrane, when the os- 

 motic pressure, now transformed into hydrostatic pressure, be- 

 comes noticeable. This distinction should be kept in mind. 



Isosmotic Coefficients. — Let us return to the work of de Vries. 

 By the plasmolytic method, he determined what solutions were 

 isotonic with the cell by measuring the concentration just nec- 

 essary to cause the first visible contraction of the protoplast from 

 the cell wall; and with the use of 0.1 molar solutions of various 

 sugars and salts (which therefore contained the same number of 

 molecules) he found that the substances used fell into four groups 

 according to the pressure developed, these pressures being 0.066, 

 0.100, 0.133, and 0.166 atmospheres, which are to each other as 

 2:3:4:5. In the first group, as shown in the following table, were 

 sucrose and glucose; in the second, potassium nitrate and ammo- 

 nium chloride, while in the last group was potassium citrate. These 

 numbers (2, 3, 4, and 5) were termed the isosmotic coefficients and 

 represent the relative osmotic pressures which are developed by 

 solutions with the same number of molecules. 



Isosmotic coefficient 

 in round numbers 



2 

 2 

 2 

 3 

 3 

 4 

 4 

 5 



It will be observed from this table that substances like glucose 

 and sucrose which have very different molecular weights exert 

 the same osmotic pressure when the same number of molecules 

 are in the solution. They thus follow the gas law and agree with 

 the explanation of osmosis as given by Van't Hoff . But the other 

 substances, — the salts, — seem to be exceptions. If they followed 

 the Van't Hoff rule, they also should give the same osmotic pres- 

 sure. 



