252 



PHYSIOLOGICAL CHEMISTRY. 



FIG. 24. Beckmann freez- 

 ing point apparatus. D is 

 a fine thermometer, C the 

 containing jar, B the out- 

 side or air mantle tube and 

 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 experimental mix- 

 ture. 



molecular weight in grams of a sub- 

 stance, such as sugar or urea, dis- 

 solved in a liter, is 1.85 below the 

 freezing-point of water. The os- 

 motic pressure of a substance of 

 which i gram molecule per liter is 

 dissolved in water, is 22.4 atmos- 

 pheres.- Therefore a freezing point 

 depression of i C. corresponds to 

 an osmotic pressure of 12.1 atmos- 

 pheres. 



Apparatus. Various forms of apparatus 

 have been devised for the experimental de- 

 termination of freezing point. The Beck- 

 mann apparatus is most commonly employed. 

 It consists essentially of a strong test-tube 

 to contain the substance to be examined. 

 This is suspended in a somewhat larger tube 

 which serves as an air bath. The large tube 

 finally is supported in a strong beaker or 

 battery jar which receives the freezing mix- 

 ture to reduce the temperature of the sub- 

 stance under experiment. The freezing 

 mixture may consist of ice, water and salt, 

 which must be stirred up frequently to main- 

 tain a uniform degree of cold. A very deli- 

 cate thermometer passes down into the sub- 

 stance in the inner tube, which is also fur- 

 nished with a stirrer of platinum wire. The 

 blood or other liquid is stirred until coagu- 

 lation begins, the thermometer being mean- 

 while carefully watched. The temperature 

 goes down at first a little below the normal 

 freezing point, because of overcooling, but 

 soon rises and remains stationary. In ex- 

 perimenting with aqueous solutions a known 

 weight of pure water is taken and its freez- 

 ing point with the thermometer used is accu- 

 rately found. Then the salt or other body, 

 which has been accurately weighed, is added 

 and a new determination made. While the 

 principle is simple the details call for some 

 skill in manipulation. Full descriptions of 

 the method may be found in works on 



