THE CHEMISTRY OF THE BODY. 



' Mercurial 'Manomeferr 



chloride solution, closed by a cork through which passes a tube attached 

 to a mercurial manometer, and immersed in distilled water (fig. 1). 

 Since the molecules of salt cannot pass through the membrane, they 

 exert pressure on the wall of the cell and the surface of the mercury ; 

 water passes through the membrane into the cell, and the mercury is 

 forced downwards in one limb of the manometer and upwards in the 

 other until the difference of height in the two limbs is 5000 mm. Hg. 

 This pressure balances the osmotic pressure exerted by the molecules 

 of salt; and in raising the column 

 of mercury the salt solution does 

 work. If a 2 per cent, salt solu- 

 tion is used, the osmotic pressure 

 is twice as great. 



Similar experiments with other 

 substances show that the osmotic 

 pressure of any substance in solution 

 depends, not upon its nature, but 

 solely on the number of its mole- 

 cules in solution, and is proportional 

 therefore to the concentration of the 

 solution. A gram - molecule of 

 any substance is its molecular 

 weight in grams. The molecular 

 weight of dextrose is 180, and 

 a gram-molecule of dextrose is 

 180 grams. A gram - molecular 

 solution of dextrose contains 180 

 grams in 1 litre, whereas a 

 similar solution of sodium chloride 

 contains 58-5 grams per litre. If 



the sodium chloride did not dissociate, the two solutions would contain 

 the same number of molecules and their osmotic pressure would be 

 the same. But since sodium chloride does dissociate and each of its 

 ions behaves like a molecule as regards osmotic pressure, the solution 

 of sodium chloride, if completely dissociated, will exert twice the osmotic 

 pressure of the solution of dextrose. 



It is difficult to measure osmotic pressure in the manner just 

 described, since the membranes are apt to give way and leak ; and 

 indirect methods are usually employed, of which the best is the 

 determination of the freezing-point of a solution. When a substance 

 is dissolved in water, the freezing-point of the water is lowered, the 

 lowering being proportional to the concentration and osmotic pressure 



I 



Inner Ifcssel (SemipermeaMe) 

 containing l%Mi.Cl. Solution. 



FIG. 1. Osmometer. (Starling's 

 Elements of Physiology. ) 



