362 A MANUAL OF PHYSIOLOGY 



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-pap (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 (Iiypoisotonic 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. 35), being taken as evidence that 

 the solution in contact with them is hypoisotonic to the contents of 

 the corpuscles. 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 appears to be 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 mole- 

 cules becomes greater. The bodies which behave in this way are 

 electrolytes that is, their solutions conduct a current of electricity ; 

 bodies which do not exhibit this behaviour do not conduct in 

 solution. And there are many reasons for believing that the dis- 

 sociation of the electrolytes is the essential thing in electrolytic 

 conduction. We may suppose that in a solution of an electrolyte 

 sodium chloride, for instance a certain number of the molecules 

 fall asunder into a kation (Na), carrying a charge of positive 

 electricity, and an anion (Cl), carrying an equal negative charge. 

 These electrical charges, it must be remembered, are not created by 

 the passage of a current through the solution. We do not know how 

 they arise, but the ions must be supposed to be electrically charged 

 at the moment when the molecule is broken up. And the ions of 

 different substances must each be supposed to carry the same 

 quantity of electricity. But since they are all wandering freely in 



