68 AN AMERICAN TEXT-HOOK OF PHYSIOLOGY. 



equal to -~2.'-','2 atmospheres. A 1 per cent, solution of cane sugar contains, however, 

 only 10 grams of sugar to a liter, hence the osmotic pressure of the sugar in such a solu- 

 tion will be ■ - of 22.32 atmospheres, or 0.G5 of an atmosphere, which in terms of a 

 342.J 



column of mercury would give 760 X 0.65 = 494 mm. This figure expresses the 

 osmotic pressure of a 1 per cent, solution of cam-sugar when dialyzed against pure 

 water through a membrane impermeable to the sugar molecules. In such an experi- 

 ment water would pass to the sugar side until the hydrostatic pressure on this side was 

 increased by an amount equal to the pressure of a column of mercury 494 mm. high. 

 Certain additional calculations that it is necessary to make for the temperature of the 

 solution need not be specified in this connection. If, however, we wished to apply this 

 method to the calculation of the osmotic pressure of a given solution of an electrolyte, 

 it would be necessary first to ascertain the degree of dissociation of the electrolyte into 

 its ions, since, as was said above, dissociation increases the number of parts in solu- 

 tion and to the same extent increases osmotic pressure. In the body the liquids that 

 concern us contain a variety of substances in solution, electrolytes as well as non- 

 electrolytes. In order, therefore, to calculate the osmotic pressure of such complex solu- 

 tions it would be necessary to ascertain the amount of each substance present, and, in 

 the case of electrolytes, the degree of dissociation. Under experimental conditions such 

 a calculation is practically impossible, and recourse must be had to other methods. One 

 of the simplest and most easily applied of these methods is the determination of the 

 freezing-point of the solution. 



Determination of Os?notic Pressure by Means of the Freezing-point. — This method 

 depends upon the fact that the freezing-point of water is lowered by substances in solu- 

 tion, and it has been discovered that the amount of lowering is proportional to the number 

 of parts (molecules and ions) present in the solution. Since the osmotic pressure is 

 also proportional to the number of parts in solution, it is convenient to take the lowering 

 of the freezing-point of a solution as an index or measure of its osmotic pressure. In 

 practice a simple apparatus (Beckmann's apparatus) is used, consisting essentially of a 

 very delicate and adjustable differential thermometer. By means of this instrument 

 the freezing-point of pure water is first ascertained upon the empirical scale of the 

 thermometer. The freezing-point of the solution under examination is then determined, 

 and the number of degrees or fractions of a degree by which its freezing-point is lower 

 than that of pure water is noted. The lowering of the freezing-point in degrees centi- 

 grade is expressed usually by the symbol A. For example, mammalian blood-serum 

 gives A = 0.56° C. A 0.95 per cent, solution of XaCl gives the same A ; hence the two 

 solutions exert the same osmotic pressure, or, to put it in another way, a 0.95 per cent. 

 solution of NaCl is isotonic or isosmotic with mammalian serum. The A of any given 

 solution may be exprc-sed in terms of a gram-molecular solution by dividing it by the 

 constant 1.87, since a gram-molecular solution of a non-electrolyte is known to lower 

 the freezing point 1.87° C. Thus if blood-serum gives A = 0.56° C, its concentration in 



0.56 

 terms of a gram-molecular solution will be T~o-, or 0.3. In other words, blood-serum 



has 0.3 of the osmotic pressure exerted by a gram-molecular solution of a non-electro- 

 lyte—that is, 22.32 x 0.3, or 6.696 atmospheres. 



Remarks upon tin Application of the Foregoing Fact* in Physiology. — In the body water 

 and substances in solution are continually passing through membranes, for example, in 

 the production of lymph, in the absorption of water and digested food-stuff's from the 

 alimentary canal, in the nutritive exchanges between the tissue-elements and the blood 

 or lymph, in the production of the various secretion-;, and so on. In these cases it is a 

 matter of the greatest difficulty to give a satisfactory explanation of the forces control- 

 ling the flow to and fro of the water and dissolved substances; but there can be little 

 doubt that in all of them the physical forces of filtration, diffusion, and osmosis take an 

 important part. Whatever can be learned therefore concerning these processes must iu 



