CHAPTER VIII 



THE THERMO-ELECTRIC METHOD OF CRYOSCOPY 



Relation between osmotic pressure and freez- 

 ing-point. The method finally adopted for determining 

 the osmotic pressures in leaves is an indirect one. 

 As is well known, a relationship exists between the 

 freezing-point of a solution and the osmotic pressure 

 it can exert against a semi-permeable membrane. Hence, 

 if we can determine the freezing-point of the sap 

 in the vacuoles we shall have a measure of the 

 osmotic pressure. This cryoscopic method has been 

 applied to determine the osmotic pressures of various 

 fluids of the animal body by numerous investigators. 

 In these cases, comparatively large quantities of the 

 fluids are available, and consequently Beckmann's method 

 for determining freezing-points is suitable. 



Beckmann's method of cryoscopy. This involves 

 the use of a thermometer with a large bulb which 

 must be immersed in the solution the freezing-point 

 of which is to be determined. The size of the bulb 

 necessitates the use of a considerable quantity of the 

 solution, viz., 12 to 15 c.c. as a minimum. 1 Such large 

 requirements seemed to preclude the application of the 

 method to the determination of the osmotic pressures of 

 the sap of transpiring organs, of which but small quantities 



1 Beckmann thermometers with diminutive bulbs have been introduced 

 recently for dealing with small quantities of liquid. 



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