5S DAVID R. BRIGGS 



accurate a value of A for the original serum as is obtainable by any 

 freezing point method. A disadvantage of the use of freezing point 

 methods, generally, when biological fluids are being examined, is the 

 fact that relatively large volumes of fluid must be employed. 



Vapor Pressure Methods. While many methods have been de- 

 vised for the determination of the molecular weights of solutes by 

 measuring their effects upon the relative vapor pressures of the 

 solvents, the accuracy claimed for these methods is in general not 

 very high even when carried out, as is required for highest accu- 

 racy, at or near the boiling points of the solvents. Such tempera- 

 tures can seldom be employed in biophysical measurements. The 

 vapor pressure method that has attained the highest degree of pre- 

 cision and one that was specifically devised for use with biological 

 tissues and fluids is the so-called thermoelectric method of Hill {15). 

 The apparatus as modified by Baldes {16) consists of a sensitive 

 thermocouple, the therm o junctions of which are made in the form of 

 loops. Upon one loop is placed a drop of the sample, e.g., blood serum 

 of unknown relative vapor pressure, and upon the other loop is placed 

 a drop of a reference salt solution (of known relative vapor pressure) , 

 which is also used to moisten a filter paper lining the air-tight chamber 

 in which the thermocouple is placed. The whole assembly is placed 

 in a water bath of constant temperature. Condensation onto (or 

 evaporation from) the drop of unknown will cause the thermojunc- 

 tion upon which it rests to become warmer (or cooler) than the other 

 junction and the temperature difference is determined by the deflec- 

 tion caused in a sensitive galvanometer connected to the leads of the 

 thermocouple. The instrument is calibrated by using a series of 

 solutions of known relative vapor pressures, placing them successively 

 on the junction later to be occupied by the unknown. The principle 

 involved is, thus, essentially the same as that of the wet bulb ther- 

 mometer in the determination of relative humidity. Roepke and 

 Baldes {17) have studied this method and emphasize that, because 

 it is a dynamic method (readings depend upon the rate of evaporation 

 or condensation upon the junctions), any or all of the following cir- 

 cumstances may lead to error in determinations in which tissues or 

 ■ body fluids are used as unknowns in an instrument calibrated with 

 simple electrolyte solutions: (a) surface films, (6) difference in coef- 

 ficient of diffusion of water in sample from that in reference solution, 

 (c) greater nonsolvent volume in sample than in reference solution, 

 {d) difference in shape of drops, owing to surface tension differences, 



