366 PHYSIOLOGICAL CHEMISTRY 



of which the temperature of the fluid under examination is lowered. 

 A large glass tube (B) designed as an air-jacket, and formed after the 

 manner of a test-tube is introduced through the central aperture in the 

 metal cover and into this air-jacket is lowered a smaller tube (A) con- 

 taining the fluid to be tested. A very delicate thermometer (D), grad- 

 uated in hundredths of a degree is introduced into the inner tube and 

 is held in place by means of a cork so that the mercury bulb is im- 

 mersed in the fluid under examination but does not come into contact 

 with any glass surface. A small platinum wire stirrer serves to keep 

 the fluid under examination well mixed while a larger stirrer is used to 

 manipulate the freezing mixture. (Rock salt and ice in the proportion 

 i : 3 form a very satisfactory freezing mixture.) 



In making a determination of the freezing-point of a fluid by means 

 of the Beckmann-Heidenhain apparatus proceed as follows: Place 

 the freezing mixture in the battery jar and add water (if necessary) to 

 secure a temperature not lower than 3C. Introduce the fluid to be 

 tested into tube A, place the thermometer and platinum wire stirrer in 

 position, and insert the tube into the air-jacket which has previously 

 been inserted through the metal cover of the battery jar. Manipulate 

 the two stirrers in order to insure an equalization of temperature 

 and observe the course of the mercury column of the thermometer very 

 carefully. The mercury will gradually fall and this gradual lowering 

 of the temperature will be followed by a sudden rise. The point at 

 which the mercury rests after this sudden rise is the freezing-point. 

 This rise is due to the fact that previous to freezing, a fluid is always 

 more or less over-cooled and the thermometer temporarily registers a 

 temperature somewhat below the freezing-point. As the fluid freezes, 

 however, there is a very sudden change in the temperature of the liquid 

 and this change is imparted to the thermometer and causes the rise as 

 indicated. It occasionally occurs that the fluid under examination is 

 very much over-cooled and does not freeze. Under such circumstances 

 a small piece of ice is introduced into it by means of the side tube noted 

 in the figure. This so-called " inoculation" causes the fluid to freeze 

 instantaneously. (For details of the method of determining the 

 freezing-point consult standard works on physical or organic chemistry.) 



Electrical Conductivity. The electrical conductivity of the urine 

 is dependent upon the number of inorganic molecules or ions present, 

 and in this differs from the freezing-point which is dependent upon the 

 total number of molecules both inorganic and organic which are in 

 solution. The conductivity of the urine has been investigated but 

 slightly, but from the data secured it seems that the value generally 

 falls below ^ = 0.03. The conductivity of blood serum has been de- 



