THE ENERGY OF MOLECULES IN SOLUTION 



139 



junction of the drop and the surrounding fluid there is a brown mem- 

 branous precipitate of copper ferrocyanide. In consequence of the 

 greater concentration of the fluid in the drop, a constant passage of 

 water takes place from without inwards through the membrane, and 

 the drop therefore grows continually in size, sometimes sending out 

 branches as a result of slight currents in the fluid set up by accidental 

 vibrations. Sugar introduced into such a drop, although quickening 

 its rate of growth, does not pass out into the surrounding copper 

 sulphate solution, nor is there any passage of copper sulphate inwards 

 or potassium ferrocyanide outwards. Pfeffer conceived the idea of 

 depositing such a semi-permeable membrane within the interstices 

 of a clay cell. Strengthened in this way, it is able to afford a resistance 

 to pressure, and therefore to permit of the contained fluid reaching 

 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 passage of water, 

 is impermeable to the sugar. The tube is then fitted with a cork 

 provided with a closed mercurial manometer 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 temperatures : 



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 Pfeffer 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 



