of the Theory of Solutions, 481 



seen that Pfeffer found the osmotic pressure of a 1 per cent, 

 sugar-solution at 6°*8 to be 0664 atmosphere. 

 We have, therefore, 



P = 0-664x10333, V = 34«2, T = 279*8. 

 Hence R=840. 



At first I thought this was a pure accident, and was dis- 

 inclined, at least in the case of sugar, to attach any physical 

 meaning to this result. Equality of both values of R, i. e. for 

 gas and solution, has only one meaning however, viz. that the 

 sugar exerts an osmotic pressure equal to the pressure which 

 gaseous sugar of the same concentration and at the same 

 temperature would exert. 



In other words, this is the application of Avogadro's law 

 to sugar-solutions, the only difference being in the substitu- 

 tion of osmotic pressure for ordinary gas-pressure. 



II. Ihe Theory of Dilute Solutions. 



Although the above remarkable equality of osmotic and 

 gas-pressure at equal molecular concentration and temperature 

 appeared at first to be a mere chance, yet it occurred again 

 and again, and was connected with so many known and after- 

 wards with newly discovered regularities, that at last there 

 seemed to be no room left for doubt. 



De Yries was the first to determine successfully the molecular 

 weights of substances in solution by an application of the 

 extended Avogadro's law. According to Loiseau-Scheibler 

 the molecule of raffinose contained 18 atoms of carbon, C l8 , 

 whilst Berthelot gave it a formula with C 2 4- In order to 

 settle the question, a comparison was made with sugar, and 

 from each substance a few isotonic solutions were prepared, 

 i. e. solutions having equal osmotic pressures. This was 

 accomplished in an extraordinary manner with the help of 

 plant-cells containing protoplasm. The membrane of the 

 protoplasm is semi-permeable, and when the cells are placed 

 in solutions having greater osmotic pressures, the protoplasm 

 contracts and separates from the cell-wall, or what is termed 

 plasmolysis takes place. If the solution has a lower osmotic 

 pressure than the contents of the cell, the protoplasm continues 

 to fill the cell completely. The point at which the protoplasm 

 just begins to recede from the cell-wall when it is placed in 

 solutions of sugar and raffinose is sought out, and the solutions 

 thus proved to be isotonic. It is then only necessary, as in 

 the case of gases, to determine the ratio of the amounts 

 of sugar and raffinose contained in equal volumes of the 



