126 



PHYSIOLOGY 



its full osmotic pressure. For this purpose a porous jar carefully cleansed 

 and containing a solution of sugar mixed with a little copper sulphate is 

 dipped into a weak solution of potassium ferrocyanide. A semi-permeable 

 membrane of copper ferrocyanide is thus produced in the pores of the filter, 

 and this, while allowing the passing of water, is impermeable to the sugar. 

 The tube is then fitted with a cork provided with a closed mercurial mano- 

 meter and is immersed in distilled water, when it is found that water passes 

 into the cell until the pressure within the latter is equal to the osmotic 

 pressure of the dissolved substances. By this means Pfeffer obtained the 

 following results with a 1 per cent, solution of cane sugar at different tem- 

 peratures : 



It is always possible to calculate the pressure of a gas when its nature, 

 its mass, and its volume are known. By Avogadro's hypothesis, equal 

 volumes of gases at the same pressure contain equal numbers of molecules. 

 On this account the molecular weight of any gas can be reckoned directly 

 from its density. The figures obtained by PfefTer show that the same 

 laws apply to the osmotic pressure of substances in solution as to the pressure 

 of gases in their free state. It is therefore possible to reckon the osmotic 

 pressure which would be exerted by 1 per cent, sugar in solution at a given 

 temperature. 



This calculation is carried out as follows : A gramme molecule of any gas at C. and 

 760 mm. Hg has a volume of 22-4 litres, therefore 342 grammes of cane sugar (the 

 molecular weight of C 12 H 22 O n = 342), if it could be converted into a gas at C. and 

 760 mm. Hg, would have a volume of 22-4 litres. One gramme of sugar therefore at 



22-4 

 the same temperature and pressure would have a volume of litres = 65-5 c.c. In 



Pfeffer's experiment the gramme of sugar was dissolved in 100 grammes of water, 

 making a total volume at C. of 100-6 c.c. The gaseous pressure of the sugar molecules 



in this solution will therefore amount to 



65-5 



= 0-651 atmosphere. At a temperature 



of 6-8 the pressure would be 0-667 atmosphere, as against the observed 0-664 atmosphere. 



Pfeffer's method is difficult to carry out and is not applicable to all 

 dissolved substances, since the cupric ferrocyanide membrane is permeable 

 for many substances, such as potassium nitrate or hydrochloric acid. Other 



