250 PHYSIOLOGICAL CHEMISTRY. 



opposite direction. Theoretically it should amount to 22.4 atmospheres 

 for a solution containing a gram molecular weight dissolved per liter, 

 since it has been found that the osmotic pressure of a body is the same 

 it would possess if it existed in the condition of a gas at the same tem- 

 perature and in the same volume. A gram molecular weight of hydro- 

 gen (2.014 gms.), of oxygen (32 gms.), of nitrogen or other gas occupies 

 a volume of 22.4 liters under normal temperature and pressure condi- 

 tions. If condensed into I liter their pressure would be 22.4 atmospheres. 

 Experiment has shown that a gram molecular weight of sugar or similar 

 solid in water to make a liter volume exerts a pressure of 22.4 atmos- 

 pheres. In the case of salts which break up into component parts or 

 ions the pressure becomes correspondingly greater. In very dilute solu- 

 tions a molecule of sodium chloride, for example, exerts practically 

 double the pressure observed for a molecule of sugar. In this dilute con- 

 dition the component parts, or ions, of sodium and chlorine seem to exert 

 a pressure corresponding to whole molecules. 



The above experiment is a somewhat crude one and is in- 

 tended merely as an illustration of the development of pres- 

 sure. For accurate measurements much more elaborate ap- 

 paratus must be employed and numerous precautions observed. 

 Practically, however, osmotic pressure is always measured by 

 indirect methods to be explained later. A familiar illustration 

 of a semi-permeable sheath or membrane is found in the red 

 blood corpuscle. Normally this holds its hemoglobin and cer- 

 tain salts because it is suspended in a liquid which has the 

 same osmotic pressure. But if the corpuscles be placed in 

 pure water they are seen to swell and finally break because of 

 the passage of water through the cell sheath which is not 

 permeable for the solid contents. By means of the hematocrit, 

 as will be explained, it is possible to find the average volume 

 occupied by the corpuscles in a given sample of blood. When 

 mixed with water or solutions with lower osmotic pressure 

 the corpuscle volume increases; in stronger salt solutions, on 

 the other hand, the individual corpuscles shrink in size and 

 their total volume becomes less. The hematocrit may there- 

 fore be used to measure or compare osmotic pressures in cer- 

 tain cases. 



INDIRECT METHODS. CRYOSCOPY. 



Although the blood contains about 20 per cent of organic 

 substances and about i per cent of mineral matters its osmotic 



