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TEXT-BOOK OF PHYSIOLOGY. 



of the sugar strike or press against the outer layer of the molecules of the solvent, 

 which at all points are in contact with the rigid walls of the earthenware vessel, 

 except at the open extremity of the vertical glass tube. Inasmuch as the rigid 

 walls of the osmometer prevent any outward displacement of the molecules of the 

 water, the force of the impact of the sugar molecule is directed against the molecules 

 at the extremity of the vertical tube which are in consequence pressed or pushed 

 upward a certain distance. Because of the loss of energy due to the impact, the 

 sugar molecule does not rebound with the same velocity, and hence time is per- 

 mitted for the molecules of the water to pass into the 

 sugar solution, to occupy the space, and thus maintain 

 the level of the fluid in the vertical tube. (For the reason 

 that the osmometer is permeable to water, the molecules 

 will pass outward as well as inward though more will 

 pass in a unit of time in the latter, than in the former 

 direction, until equilibrium is established.) The pressure 

 of the sugar molecules continuing, the level of the fluid in 

 the glass tube continues to rise and the level of the fluid in 

 the vessel, b, continues to fall until the force of gravity 

 prevents any further upward movement of the molecules 

 of sugar against the outer film of the molecules of the 

 water. The difference in the level of the two fluids ex- 

 pressed in millimeters of mercury is taken as a measure 

 of, and equal to, the pressure of the sugar in solution. 

 A i per cent, solution of cane-sugar at a temperature 

 of from 13 C. to 16 C., as determined by this method, 

 exerts an osmotic pressure of about 535 mm. Hg.; a 2 

 per cent, solution exerts an osmotic pressure approxi- 

 mately twice this amount. 



Experiments made with this and similar osmometers 

 show 



1. That the osmotic pressure of any substance in solu- 



tion is proportional to the concentration, providing 

 the temperature is constant. 



2. That when the concentration is constant the osmotic 

 pressure rises with, and is proportional to, the tempera- 

 ture. 



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. 

 .. 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; 

 weights in grams per liter of different substances 

 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 methods 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- 



Solution 



f 

 Cane Sugar 



hence the molecular 



