260 



PHYSIOLOGICAL CHEMISTRY. 



lected. By this means considerable aid in the diagnosis of renal 



diseases may be secured. The fluids most frequently examined 



cryoscopically are the blood (seep. 182) and 



FIG. 84. 



the urine. The freezing-point is denoted 



by A. The value of A for normal urine 

 varies ordinarily between 1.3 and 2.3 

 C., 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 

 0.2 C. whereas on a diet containing much 

 salt and deficient in fluids the value of A 

 may be lowered to 3 C. or even lower. 



\ The freezing-point of normal blood is gen- 



erally about 0.56 C. and is not subject to 

 the wide variations noted in the urine, be- 

 cause of the tendency of the organism to 

 maintain the normal osmotic pressure of the 

 blood under all conditions. Variations be- 

 tween 0.51 and 0.62 C. may be due 

 entirely to dietary conditions 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-Heiden- 

 hain apparatus (Fig. 84, p. 260) or the Zikel 

 Pektoscope. The Beckmann - Heidenhain 

 apparatus consists of the following parts : 

 A strong battery jar or beaker (C) fur- 

 nished with a metal cover which is provided 

 with a circular hole in its center. This 

 strong glass vessel serves to hold the freez- 

 ing mixture by means of which the tem- 

 perature of the fluid under examination is 

 lowered. A large glass tube (B) designed 

 as an air-jacket, and formed after the man- 

 ner of a test-tube is introduced through the 

 central aperture in the metal cover and into this air-jacket is low- 

 ered a smaller tube (A) containing the fluid to be tested. A very 



BECKMANN-HEIDENHAIN 

 FREEZING-POINT APPA- 

 RATUS. (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 mix- 

 ture in the jar and one 

 for the experimental mix- 

 ture. 



