PHYSICAL METHODS IX BLOOD EXAMINATION. 253 



physical or organic chemistry, as it finds applications in many directions, 

 and especially in the determination of molecular weight. The general 

 appearance of the simple apparatus is here shown. 



The freezing point of normal human blood is about 



0.56. As a reduction of i corresponds to an atmospheric 

 pressure of 12.1 atmospheres, the normal osmotic pressure of 

 the blood is about 6.8 atmospheres. It makes but little differ- 

 ence here whether we consider the whole blood or the plasma 

 free from corpuscles and fibrin. The result is mainly due to 

 the small molecules present, and these are inorganic. A solu- 

 tion of 20 gms. of serum albumin in water to make 100 cc. 

 would have a freezing point of about 0.03; the effect of 

 the other proteins would be practically the same. A solution 

 of urea containing 10 gm. in 100 cc. has a freezing point of 

 3-oS 3 , one of glucose with 10 gm. in 100 cc. a freezing 

 point of i -03, while a solution of common salt with the 

 same weight dissolved would show a depression of about 5. 



Variations. This observed freezing point depression is 

 normally constant and nearly the same for the blood of all the 

 common animals. But temporary variations may occur. 

 After consumption of large quantities of water it may sink to 



0.51, while following a meal rich in salty food a further 

 depression to 0.62, or even lower, may be observed. But 

 these changes are very speedily rectified through the elimina- 

 tion of proper quantities of salts and water by the kidneys. 

 If an examination of the blood shows a greater depression than 

 that which may be accounted for by absorption of food con- 

 stituents a failure of some kind in the functions of the kidneys 

 is indicated. Through injury to the mechanism of these or- 

 gans the osmotic pressure of the blood may rise to over 12 

 atmospheres, corresponding to a depression of the freezing 

 point of a whole degree or more. 



Because of these observed facts the determination of the 

 freezing point of the blood has become a test of practical im- 

 portance in the diagnosis of disorders of the kidney. With 

 proper facilities the experiment may be quickly made and will 



