Chapter VII 



95 



Osmotic Quantities of Cells 



proximately 1° C. This procedure requires a preliminary trial to determine where 

 freezing must begin to be 1° C. below the freezing point. 



As proof of the validity of correcting observed freezing point values as described, 

 freezing curves, such as that shown in Figure 31, were constructed according to the 

 recommendations of Mair, Glasgow, and Rossini (1941). Extrapolation of the part 

 of the curve where thermodynamic equilibrium exists between the solid and liquid 

 phases of the system, back to the initial downward part of the curve, gives a close 

 approximation of the true freezing point. The above authors point out that in a strict 

 sense the true freezing point is slightly higher than this, but they have proved that 

 the difference is insignificant when the amount of supercooling is small in comparison 

 with the head of temperature (difference between the temperature of freezing bath and 

 that of the test solution). With a head of 10° to 15° C, this error was found to be 

 insignificant in the determinations made as described. True freezing points of both 

 sucrose solutions and beet juice, obtained by plotting freezing curves, agree with those 

 corrected by means of the calibration curve of Figure 30. 



Fig. 31. — Freezing curve constructed according to recommendations 

 of AIair, Glasgow, and Rossini (1941). Extrapolation of the latter 

 portion back to the initial downward line gives a close approximation of 

 the true freezing point of the solution. 



In dealing with plant saps of high viscosity, separation of ice may occur more 

 slowly, resulting in a slower rise of the mercury column and a greater proportion of 

 heat lost to the system. Under these circumstances a calibration curve prepared by use 

 of sucrose may not be valid. The error would likewise not be corrected by use of the 

 formula for supercooling. Where doubt exists, comparison of freezing curves like that 

 of Figure 31 with similar curves for sucrose solution will indicate the error involved. 



Vapor Pressure Methods : — A unique method for determining 

 osmotic pressure is the vapor pressure method devised by Barger (1904, 

 1924) for the purpose of calculating molecular weights, and adapted by 

 Halket (1913) to plant saps. Small glass capillaries of uniform bore 



