ABSORPTION OF MATERIALS IN GENERAL 



109 



In the above table the isosmotic coeflicients are seen to be about 2, 3, 4 

 and 5. If this coefficient for saccharose and the other organic compounds be 

 taken as unity, then the remaining ones become ^, 2, and ^. 



It is also evident from this table that the osmotic pressures produced by the 

 non-electrolytes (saccharose, glycerine and the other organic compounds) are re- 

 lated to their molecular weights. A solution containing 92 g. of glycerine per 

 liter produces the same osmotic pressure as one of cane sugar containing 342 g. 

 per liter. These two solutions contain very different amounts of substance 

 by weight, but they contain equal numbers of molecules {i.e., they are equi- 

 molecular). Here all molecules produce the same osmotic pressure, and the 

 osmotic pressure of a solution is thus proportional to its molecular concentra- 

 tion. This agrees with Avogadro's law for gases, which states that gas pressure 

 is proportional to the number of molecules occurring in a given volume. Van't 

 Hoff compared solutions of solid bodies in liquids, with gases, and concluded 

 that osmotic pressure follows the same law as does gas pressure. One gram- 

 molecule of any gas (e.g., 44 g. of CO2) occupies a volume of 22.4 1., with a pres- 

 sure of 760 mm. and at a temperature of o°C. When this volume of gas is re- 

 duced to I 1., the pressure becomes 22.4 atmospheres. If the van't Hofif theory 

 is correct, a molecular solution of cane sugar containing 342 g. per liter, should 

 produce 22.4 atmospheres of osmotic pressure, and a i-per cent, solution of 

 the same substance should give an osmotic pressure of o.6g atmospheres at 

 iS°C. The pressure actually produced by a i-per cent, solution of cane sugar 

 lies between 0.62 and 0.71 atmospheres according to Pfefier's measurements, 

 which constitutes a brilliant confirmation of the theory. 



The following table gives a summary of other osmotic values for cane- 

 sugar solutions, as observed by Pfeffer and as calculated by the van't Hoff 

 theory. 



It is different with electrolytes; from the table given on page 108 it is clear 

 that, of the crystalloids, isosmotic solutions of electrolytes (metallic salts) and 

 non-electrolytes are not equimolecular, the molecular concentrations of the 

 former being much lower. Furthermore, there is no constant relation between 

 the isosmotic concentrations of solutions of electrolytes on the one hand and of 

 non-electrolytes on the other, so that electrolytes do not agree with the gas- 

 pressure theory of osmotic pressure. For example, a o.i-volume-molecular solu- 



