228 TEXT-BOOK OF PHYSIOLOGY 



3. That when different substances are present in the same solvent the osmotic 



pressure is equal to the sum of the individual or partial pressures. 



4. That whatever the nature of the substance in solution it will exert the same 



osmotic pressure, providing always the same number of molecules are 

 present; hence the molecular weights in grams per liter of different sub- 

 stances exert the same osmotic pressure at the same temperature. 



Because of the fact that when certain substances, e.g., many inorganic salts, 

 many acids and bases, are dissolved, some of their molecules undergo ionization, 

 i.e., separation into parts which are charged with electricity, and hence the two 

 together, molecules and ions, exert a greater osmotic pressure than would other- 

 wise be the case; and because of the further fact, that it is extremely difficult to 

 obtain absolutely semipermeable membranes, uniform results are not obtained by 

 the employment of the three methods; therefore, the osmometric metheds as well 

 as the calculation or arithmetic method have been largely discarded and the 

 method based on the determination of the freezing point has been adopted. 



2. The Determination of the Freezing Point. Because of the difficulty in obtain- 

 ing the exact osmotic pressure by means of the osmometer as stated above, reliance 

 is now placed on the mathematic relation known to exist between osmotic pressure 

 and the freezing point. Thus the freezing point of water holding any substance 

 in solution is lower than water itself and is indeed proportional to the number 

 of molecules dissolved. As a standard of comparison it is customary to employ 

 a gram-molecule of a substance dissolved in one liter of water. (A gram-molecule 

 is the quantity of a substance expressed in grams equal to its molecular weight.) 

 The lowering of the freezing point of a gram-molecule solution below that of water 

 is constant, viz., i.87C. The osmotic pressure therefore of such a solution, as 

 determined by calculation (see below), is equal to 22.38 atmospheres, or 17,008 

 mm. of Hg. 



Therefore it is only necessary to determine by means of a differential thermome- 

 ter the lowering of the freezing point in degrees centigrade, which is usually expres- 

 sed by the symbol A. Then the osmotic pressure is equal to A divided by 1.87 

 C., and multiplied by 22.38 atmospheres, or 17,008 mm. of Hg. Thus if the 

 freezing point of any solution was found to be o.83C. lower than water, its 

 osmotic pressure would be 0.83-^1.87X22.38 atmospheres or 9.847 atmospheres 

 = 7,483 mm. Hg. If any two solutions have the same freezing point they contain 

 the same number of molecules and hence have the same osmotic pressure. Blood 

 plasma has a freezing point of o.56C. Experimentally it has been determined 

 that the freezing point of water is lowered to the same level, when it contains so- 

 dium chlorid to the extent of 0.95 per cent. Hence these two fluids have the same 

 osmotic pressure and are isotonic; each exerts a pressure of 6.696 atmospheres. 



For this reason the sodium chlorid solution can be employed for preserving, 

 for a time at least, the form of blood corpuscles or other living mammalian cells, 

 from which it may be inferred, that the contents of the cells have an osmotic pres- 

 sure approximately equal to that of the blood plasma or the salt solution. If the 

 salt solution has a lower concentration and hence a lower osmotic pressure, water 

 will osmose into the corpuscle and cause a discharge of its hemoglobin content. 

 Such a fluid is said to be hypo-isotonic. If, on the contrary, the salt solution has a 

 higher concentration and hence a higher osmotic pressure, water will osmose from 

 the corpuscle causing a shrinkage and crenation of the corpuscle. Such a fluid 

 is said to be hyperisotonic. 



3. By Calculation. The osmotic pressure may also be obtained by calculation 

 based on the known pressure exerted by a gram-molecule of hydrogen 2 grams 

 when compressed to a volume of one liter. It is well known that i gram of hydro- 

 gen at oC. and at an atmospheric pressure of 760 mm. Hg. occupies a volume of 

 11.19 liters, and that 2 grams under the same conditions will occupy a volume of 

 22.38 liters, and that when the two grams, that is, one gram-molecule is com- 



