CELLS AND TISSUES. 39 



nicity of a single solution is impossible, as KOPPE has well 

 pointed out by indicating the confusion wrought by this 

 expression. The osmotic effect of a solution may be 

 expressed in terms of molecular concentration (mol.). The 

 molecular weight of a non-dissociable substance (such 

 as cane-sugar), expressed in grams and dissolved in 

 enough water to make a liter, constitutes the unit of 

 molecular concentration. Such a solution has an osmotic 

 pressure of 22.35 atmospheres at o C. Isotonic solutions 

 are equimolecular. 



The fact that there exists a difference between physical 

 and physiological determinations of osmotic pressure is 

 emphasized all too little. In the former case the pressure 

 is measured against the solvent, in the second case the 

 pressure of a solution against cells or their contents. 

 Each of these values, differing as it does more or less 

 from the other, may have its own biological signifi- 

 cance. 



A study of the changes in the volume of cells brought 

 about through differences in osmotic pressure clearly 

 shows that the conditions for the unlimited tenability of 

 VAN'T HOFF'S laws do not exist in the living organism. 

 In its simplest form VAN'T HOFF'S theory presupposes 

 two things: impermeability of the separating membrane 

 for the dissolved substance, and complete freedom of 

 movement of the solvent throughout the entire medium 

 contained within the membrane. If these two conditions 

 existed in the case of cells, then two series of facts should 

 be found to be true experimentally : 



I. A cell should have the same volume in isosmotic 

 solutions of different substances. 



II. A cell should show an amount of change in volume 



