206 ESSENTIALS OF CHEMICAL PHYSIOLOGY 



Determination of Osmotic Pressure by means of the Freezing-point.— This is 

 the method which is almost universally employed. A very simple apparatus 

 (Beckmann's differential thermometer) is all that is necessary. The principle 

 on which the method depends is the following : — The freezing-point of any 

 substance in solution in water is lower than that of water ; the lowering of 

 the freezing-point is proportional to the molecular concentration of the dis- 

 solved substance, and that, as we have seen, is proportional to the osmotic 

 pressure. 



When a gramme-molecule of any substance is dissolved in a litre of water, 

 the freezing-point is lowered by 1-87° C, and the osmotic pressure is, as we have 

 seen, equal to 22*38 atmospheres, that is, 22"38 x 760 = 17,008 mm. of mercury. 



We can, therefore, calculate the osmotic pressure of any solution if we 

 know the lowermg of its freezing-point in degrees Centigrade ; the lowering 

 of the freezing-point is usually expressed by the Greek letter A. 



Osmotic pressure = —— x 17,008 



For example, a 1-per-cent. solution of sugar would freeze at -0-052° C. ; 



■ ,. f -052 X 17,008 .„o V, 



its osmotic pressure is thereiore ?Tq7^ ~ '*'^ mm., a number approxi- 

 mately equal to that we obtained by calculation. 



Mammalian blood serum gives A = 0-56° C. A 0-9-per-cent. solution of so- 

 dium chloride has the same A ; hence serum and a 0*9-per-cent. solution of com- 

 mon salt have the same osmotic pressure, or are isotonic. The osmotic pressure 



Pill • '56 X 17,008 . narr j? « 



of blood serum is =4,987 mm. or mercury, or a pressure of 



1'87 



nearly 7 atmospheres. 



The osmotic pressure of solutions maj- also be compared by observing 

 their effect on red corpuscles, or on vegetable cells such as those in Trades- 

 cantia. If the solution is hypertonic, i.e. has a greater osmotic pressure 

 than the cell contents, the protoplasm shrinks and loses water, or if red cor- 

 puscles are used, they become crenated. If the solution is Jiy2}otonic, e.g. has 

 a smaller osmotic pressure than the material within the cell-wall, no shrink- 

 ing of the protoplasm in the vegetable cell occurs, and if red corpuscles are 

 used they swell and liberate their pigment. Isotonic solutions produce 

 neither of these effects, because they have the same molecular concentration 

 and osmotic pressure as the material within the cell-wall. 



Physiological Applications.— It will at once be seen how important all these 

 considerations are from the physiological standpoint. In the body we have 

 aqueous solutions of various substances separated from one another by 

 membranes. Thus we have the endothelial walls of the capillaries separating 

 the blood from the lymph ; we have the epithelial walls of the kidney tubules 

 separating the blood and lymph from the urine ; we have similar epithelium 

 in all secreting glands ; and we have the wall of the alimentary canal 

 separating the digested food from the blood-vessels and lacteals. In such 

 important problems, then, as lymph -formation, the formation of urine and 

 other excretions and secretions, and absorption of food, we have to take into 

 account the laws which regulate the movements both of water and of sub- 

 stances which are held in solution by the water. In the body osmosis is not 



