400 A MANUAL OF PHYSIOLOGY 



copper. 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. I ts osmotic 



pressure = ^^ 4 x 16,986 = 493 mm. of mercury. 



A convenient apparatus for making freezing-point measurements 

 is shown in Fig. 153. The details of the method are given in the 

 Practical Exercises, p. 492. 



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 protoplasm 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 

 liberation of haemoglobin or the swelling of the corpuscles, as 

 measured by the haematocrite (p. 59), 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 propor- 

 tional 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 



