PHYSICAL METHODS IX BLOOD EXAMINATION. 251 



pressure depends largely on the latter. This is because of the 

 simple fact that the large gross weight of organic matter rep- 

 resents relatively but a small number of molecules, and the 

 actual pressure is measured by the total number of molecules 

 or ions present. .This osmotic pressure in health remains 

 practically constant; even after great loss of blood when the 

 total volume is restored by drawing on the lymph serum, al- 

 though the relative .number of corpuscles may be much re- 

 duced, the osmotic pressure of the new blood is practically 

 unchanged. This is due to the fact that the blood and lymph 

 serum are isosmotic. 



The Freezing Point Method. The measurement of the 

 osmotic pressure of blood or any other solution by the direct 

 method suggested by the experiment given above is extremely 

 difficult. Several indirect methods may be followed, two of 

 which are in common application. One of these only is suit- 

 able for the examination of blood. In the first of these meth- 

 ods the elevation of the boiling point of the solution is ob- 

 served; in the second the depression of its freezing point. 

 Comparatively simple relations obtain between the three phe- 

 nomena. In a solution the tension of the vapor is decreased 

 in proportion as the osmotic pressure of the dissolved sub- 

 stance increases and more heat must therefore be applied to 

 actually lift the atmosphere or boil off the solvent. For each 

 gram-molecule per liter dissolved this elevation of the boiling 

 point of water is about 0.52. This method, by noting the 

 elevation of the boiling point, cannot be applied to blood, be- 

 cause of its coagulation, but there is no drawback to the 

 method depending on the separation of the solvent by freez- 

 ing. With increase in amount of salt or foreign substance 

 dissolved in the water, the lower must its temperature be 

 brought to effect a partial separation by freezing out a portion 

 of the solvent. The lowering of the freezing point is accu- 

 rately proportional to the number of molecules (or ions) pres- 

 ent. The molecular freezing point depression for water is 

 1.85; that is the freezing point of a solution containing one 



