II. OSMOTIC PRESSURE MEASUREMENTS 59 



(e) difference in heat of condensation, (/) thermochemical reactions 

 in sample (particularly in intact cells), and (g) presence of volatile 

 solutes. While the possible importance of these sources of error will 

 vary with the nature of the sample being studied, these authors con- 

 cluded that, if the "osmotic pressure of a sample of blood differed 

 from that of the reference solution by 10 per cent, the osmotic pressure 

 as determined with the thermocouple would be in error by less than 

 0.2 per cent." 



This method, in the hands of a careful and meticulous worker, 

 probably constitutes the best available means for determination of 

 the osmotic pressure of solutions, containing low molecular constit- 

 uents, when the determination must be made on the solution without 

 change in concentration and at temperatures above the freezing 

 point of the solvent. For example, the state of aggregation or dis- 

 sociation of detergents in water solution is a function both of the 

 concentration and of the temperature of the solution, there existing 

 both molecular and micellar components of the detergent in the solu- 

 tion. Measurements made with the freezing point depression method 

 yield information with regard to the state of the solute only at or near 

 the freezing temperature of the solvent. Effects of change in tem- 

 perature of the solution upon the dissociation-association properties 

 of the solute cannot be studied by this method. Direct osmotic 

 pressure measurements are very difficult to make on such systems 

 because of the difficulty in obtaining semipermeable membranes that 

 will be impermeable to the low molecular components. This vapor 

 pressure method can be employed on such systems with considerable 

 success (18). It is quite probable that similar variations with con- 

 centration and temperature may obtain in biological systems. This 

 method, when the measurements are made at body temperature, 

 eliminates the possibility of errors arising from these possible varia- 

 tions. 



2. Direct Methods 



Plasmolysis Method. Where isolated cells or small tissue 

 specimens are being investigated, the simplest and most direct 

 method for determining their osmotic properties is by the plasmol- 

 ysis method commonly attributed to de Vries. By placing speci- 

 mens in a series of solutions of varying concentrations of some solute 

 or mixture of solutes, the osmotic pressures of these solutions being 

 known, and observing, with a microscope, which solutions cause the 



