Dandeno: Osmotic theories 289 



this : '* The determining factor in osmotic pressure is the numerical 

 ratio existing between the molecules of the dissolved substance 

 and of the solvent, rather than the number of molecules of the dis- 

 solved substance which are contained in a unit volume of the solu- 

 tion." If we place in a tube the three liquids as above, but instead 

 of pure water we substitute a water solution of iodine with potassic 

 iodide, in a day or two the brownish solution in the center becomes 

 colorless towards both top and bottom, while the chloroform and 

 the xylol have turned reddish, showing that the iodine has passed 

 into these Hquids. Now if the dissolved iodine acted as a gas, we 

 should have the water solution becoming lighter-colored, but uni- 

 formly so, and not light-colored at each end. When the iodine 

 left the water to pass into the chloroform, other molecules of iodine 

 would replace them from behind, if they were driven into the 

 chloroform by '* gas-pressure," Several other experiments with 

 the liquids, also with turpentine, xylene, ether, and carbon disul- 

 phide have been made with colored substances in solution ; and 

 the conclusion reached from observation of these is that the pres- 

 sure which we call osmotic pressure is due to attraction. 



The Traube * pellicle affords a further illustration of the case 

 in point. When the lump of CuClg is dropped into the solution of 

 potassium ferrocyanide a membrane is formed about the lump. In- 

 side of this membrane is CuClg solution, and outside is the ferro- 

 cyanide solution. The water is taken in through the membrane 

 and into the CuCl^ solution, thus Increasing the volume. The 

 membrane bursts and is immediately repaired. The process is 

 continued with another rupture, and so on. Now, the point of 

 rupture is always at, or near, the top of the sack of CuCl^ solu- 

 tion. The solution of copper chloride is much more dilute at the 

 top, yet, if the theory of *' bombardment of molecules" were sound, 

 it should rupture obviously where the molecules are most numer- 

 ous, which is at the bottom. It can easily be seen that the solu- 

 tion is more dense at the bottom by the deep blue color, and it 

 would naturally be so because of the very slow diffusion of the 

 solute upwards. As has been said, the sack should rupture at 



w 



the bottom, assuming the "gas-pressure" theory. The reason 

 the sack ruptures at the top — an explanation not necessary, how- 



* Described in Pfeffer, Physiologj' of plants i : io6. 1900 [trans, by EvvartJ. 



