382 



PHYSIOLOGICAL CHEMISTRY 



This coefficient was calculated on the basis of the specific gravity deter- 

 mined at a temperature of i5C. 



Freezing-point (Cryoscopy). The freezing-point of a solution de- 

 pends upon the total number of molecules of solid matter dissolved in 



it. The determination of the osmotic pressure 

 by this method has come to be of some clinical 

 importance, particularly as an aid in the diag- 

 nosis of kidney disorders. In this connection 

 it is best to collect the urine from each kidney 

 separately and determine the freezing-point in 

 the individual samples so collected. By this 

 means considerable aid in the diagnosis of renal 

 diseases may be secured. The fluids most fre- 

 quently examined cryoscopically are the blood 

 (see page 248) and the urine. The freezing- 

 point is denoted by A. The value of A for 

 normal urine varies ordinarily between 1.3 

 and 2.3C., the freezing-point of pure water 

 being taken as o. A is subject to very wide 

 fluctuations under unusual conditions. For 

 instance, following copious water- or beer- 

 drinking A may have as high a value as 



o.2C., whereas on a diet containing much 

 salt and deficient in fluids the value of A may 

 be lowered to 3C. or even lower. The freez- 

 ing-point of normal blood is generally about 



o .56C. and is not subject to "the wide 

 variations noted in the urine, because of the 

 tendency of the organism to maintain the 

 normal osmotic pressure of the blood under all 

 conditions. Variations between 0.51 and 

 o.62C. may be due entirely to dietary con- 

 ditions, but if any marked variation is noted 

 it can, in most cases, be traced to a disordered 

 kidney function. 



Freezing-point determinations may be made 

 by means of the Beckmann-Heidenhain appa- 

 ratus (Fig. 122) or the Zikel pektoscope. The 

 Beckmann-Heidenhain apparatus consists of 

 the following parts: A strong battery jar or beaker (C) furnished 

 with a metal cover which is provided with a circular hole in its center. 

 This strong glass vessel serves to hold the freezing mixture by means 



FIG. 122. BECKMANN- 

 HEIDENHAIN FREEZING- 

 POINT APPARATUS. (Long.) 



D, a delicate thermom- 

 eter; C, the containing 

 jar; B, the outside or air 

 mantle tube; A, the tube in 

 which the mixture to be 

 observed is placed. Two 

 stirrers are shown, one for 

 the cooling mixture in the 

 jar and one for the experi- 

 mental mixture. 



