THE ENERGY OF MOLECULES IN SOLUTION 



125 



DETERMINATION OF OSMOTIC PRESSURE BY PLASMOLYSIS. 



Solutions which have the same osmotic pressure are spoken of as isosmotic or isotonic. 

 The method of plasmolysis, which we owe to the botanist De Vries, consists essentially 

 in the comparison of the osmotic pressure of solutions with that of the cell sap of certain 

 plant cells, and depends on the fact that the ' primordial utricle,' the layer of protoplasm 

 enclosing the cell sap, while freely permeable to water, is impermeable to a large number 

 of salts and other crystalloids, such as sugar. It is therefore, so far as concerns these 

 substances, * semi-permeable.' The cells which have been most used for this purpose 

 are the cuticular cells on the mid-rib of the lower surface of the leaves of tradescantia 

 discolor. If some of these cells are brought into a concentrated salt solution, which is 

 * hypertonic ' as compared with the cell sap, water passes out of the cell into the salt 

 solution, until the contents of the cell attain a molecular concentration equal to that 

 of the surrounding medium. The protoplasmic layer therefore shrinks, leaving a space 

 between it and the cell wall (Fig. 7, p. 23). If the outer solution has a smaller molecular 

 concentration than the cell sap, water passes into the cell and causes here a rise of 

 pressure which simply presses the protoplasm still more closely against the cell wall. If 

 we determine the concentration of the salt solution at which the shrinkage of the proto- 

 plasm, the plasmolysis, just occurs, and another smaller concentration at which plas- 

 molysis is absent, we know that the concentration of the cell sap lies between those 

 of the two salt solutions. Thus, if plasmolysis occurs in a solution containing 0-6 

 per cent, sodium chloride and is absent in a solution containing O59 per cent, of the 

 same salt, the concentration of the cell sap must be about equivalent to a 0*595 per cent. 

 NaCl solution. Solutions of different salts, in which plasmolysis just occurs, must also 

 be isotonic with one another. Thus a 1-01 per cent, solution of KN0 3 is found to be 

 isotonic with a O58 per cent. NaCl solution. 



DETERMINATION BY HAMBURGER'S BLOOD-CORPUSCLE METHOD. 

 The limiting external layer of red blood corpuscles resembles the primordial utricle of 

 plant cells in being impermeable to a number of dissolved substances. If therefore 

 it be placed in a solution of smaller concentration than the corpuscle contents, it will 

 swell up and, since it has no supporting cell wall, the increase in size will go on until 

 the corpuscle bursts, and its contained red colouring-matter, haemoglobin, passes into 

 solution in the surrounding fluid. If the corpuscles be then allowed to settle or be 

 centrifuged, the fact that haemolysis has occurred is shown by the red colour of the 

 clear supernatant fluid. With a given sample of blood, the concentration of a potassium 

 nitrate solution is found at which the first traces of haemolysis occur. In order to 

 determine the osmotic pressure of a solution, say, of sugar or of sodium chloride, these 

 are also added in various dilutions to blood corpuscles until we get solutions in which 

 haemolysis just occurs. These solutions will then be isotonic with the first determined 

 potassium nitrate solutions. As an example of this method may be adduced the 

 following results : 



