OSMOTIC PRESSURE n 



general the osmotic strength of a solution depends upon the num- 

 ber of molecules it contains (though this does not apply to dilute 

 solutions of many salts), so that when these are present in equal 

 proportions, two solutions (e.g. of cane-sugar and grape-sugar) 

 have the same osmotic pressure. Such equimolecular solutions 

 are obtained by dissolving substances, in a litre of distilled 

 water, in proportions equivalent to their molecular weights. 

 If the number of grammes of the compound dissolved in a litre 

 of water is equivalent to the molecular weight, we have a so-called 

 molecular solution, briefly indicated by M. Solutions of other 

 strengths are indicated as o'5 M, 0*2 M, etc. Since the weight 

 of a substance depends on the number and mass of the atoms 

 composing its molecule, a 10 per cent, solution of a complex com- 

 pound, such as inulin or dextrin, will contain fewer molecules 

 than a 10 per cent, solution of a simpler compound, such as 

 grape-sugar or cane-sugar ; moreover, cane-sugar, which has a 

 higher molecular weight than grape-sugar, will exhibit a lower 

 osmotic pressure than the latter when in solutions of the same 

 percentage strength. These important facts are illustrated in 

 the following table : 



Molecular Molar concentration Osmotic pressure 



weight, of 10 per cent, solution. in atmospheres. 



Grape-sugar . . 180 0*555 12*43 



Cane-sugar . . 342 0*292 6*54 



Dextrin ... . ca. 2*2 



Albumen . . ca. 13,000 0*008 0*17 



In order to determine the osmotic pressure of any cell, a 

 solution of sea-water (see Appendix III), strong enough to bring 

 about plasmolysis, is first obtained. We next proceed to find 

 a slightly weaker solution such as will fail to plasmolyse the 

 cell. By experimenting with a series of solutions of intermediate 

 strengths, one can eventually be found which causes very little 

 contraction of the protoplast, the plasmatic membrane only 

 receding very slightly from the angles of the cell. The osmotic 

 strength of the solution which produces this effect is somewhat 

 stronger than that of the cell-sap, since, at first, the contraction 

 of the protoplast is accompanied by shrinkage of the stretched 

 cell-wall. The osmotic pressure of the sap may, however, be 

 taken as approximately that of the solution which just fails to 



