428 ABSORPTION 



It is as yet impossible or at least very difficult to directly measure 

 the osmotic pressure with accuracy by means of a semi-permeable mem- 

 brane. Recourse is therefore had to indirect methods, especially one 

 which depends on the fact that the freezing-point of a solution is lower 

 than that of the solvent, salt water, e.g., freezing at a lower temperature 

 than fresh water. The amount by which the freezing-point is lowered 

 depends on the molecular concentration of the dissolved substance, to 

 which, as we have seen, the osmotic pressure is also proportional. When 

 a gramme-molecule of a substance is dissolved in water, and the volume 

 made up to a litre, the freezing-point is lowered by r86 C. ; the osmotic 

 pressure is 22-35 atmospheres (16,986 mm. of mercury). It is therefore 

 easy to calculate the osmotic pressure of any solution if we know the 

 amount by which its freezing-point is lowered. A i per cent, solution of 

 cane-sugar, for example, would freeze at about 0-054 C. Its osmotic 



pressure = ,^x 16,986=493 mm. of mercury. 



A convenient apparatus for making freezing-point measurements is 

 shown in Fig. 171. The details of the method are given in the Practical 

 Exercises, p. 529. 



The osmotic pressure of different solutions may also be compared 

 by observing the effect produced on certain vegetable and animal cells. 

 When a solution with a greater osmotic pressure than the cell-sap (a 

 hyperisotonic solution) is left for a time in contact with certain cells in 

 the leaf of Tradescantia discolor, plasmolysis occurs that is, the proto- 

 plasm loses water and shrinks away from the cell-wall. If the osmotic 

 pressure of the solution is lower than that of the coloured cell-sap 

 (hypoisotonic solution), no shrinking of the protoplasm takes place. By 

 using a number of solutions of the same substance but of different 

 strength, two can be found, the stronger of which causes plasmolysis, 

 and the weaker not. Between these lies the solution which is isotonic 

 with the cell-sap that is, has the same molecular concentration and 

 osmotic pressure. The strength of an isotonic solution of some other 

 substance can then be determined in the same way with sections from 

 the same leaf. 



Animal cells (red blood-corpuscles) may also be employed, the libera- 

 tion of haemoglobin or the swelling of the corpuscles, as measured by 

 the haematocrite (p. 27), being taken as evidence that the solution in 

 contact with them is hypoisotonic to the contents of the corpuscles. 

 Here we may suppose that the impacts of the molecules of the salts of 

 the corpuscle on the inside of its envelope, not being balanced by 

 similar impacts on the outside, tend to distend it, and thus to create a 

 potential vacuum for the surrounding water, which accordingly enters. 

 If the corpuscles shrink, the solution is hyperisotonic to their contents. 

 But since the cells are much more permeable to certain substances than 

 to others, this method does not always yield trustworthy results. 



Electrolytes. We have said that the osmotic pressure is proportional 

 to the concentration of the solution, but this statement must now be 

 qualified. For certain compounds, including all inorganic salts and 

 many organic substances, the osmotic pressure decreases less rapidly 

 than the theoretical molecular concentration as the solution is diluted. 

 The explanation is that in solution some of the molecules of these bodies 

 are broken up into simpler groups or single atoms, called ions. Each 

 ion exerts the same osmotic pressure as the molecule did before. The 

 proportion between the average number of these dissociated molecules 

 and of ordinary molecules is constant for a given concentration of the 

 solution and a given temperature. But as the solution is diluted, the 

 proportion of dissociated molecules becomes greater. The bodies which 



