8 Mr. P. S. Barlow: Osmotic Experiments 



A cell was soaked in methyl alcohol, the outer vessel being 

 closed by a cork covered with melted wax. With changes 

 of alcohol, both sides were obtained osmotically similar ; that 

 is, the closed cell showed neither increase nor decrease of 

 pressure. This being so, the cork of the outer vessel was 

 fitted with a water manometer. The absorption of the water 

 vapour by the alcohol caused a slow but steady increase of 

 the internal pressure. In the case of ethyl alcohol exposed 

 to the air of the laboratory, the same result was obtained. 

 In these cases, it might have been thought on a 'priori grounds 

 that the alcohol was in so very large excess, that it could 

 have been regarded as dissolving the water. Experimentally, 

 the water acted as the solvent (in the osmotic sense) . 



The explanation of the temporary effects is very simple. 

 After the preparation of the cell in the ordinary way, the 

 cell-wall is soaked with water. When the solution is within 

 the cell, the membrane soon gets into what may be called the 

 osmotic condition ; that is, the condition of having water at 

 its outer surface, and solution at its inner surface. The 

 normal inflow of water takes place. 



When placed in alcohol, the outer face of the membrane 

 remains in contact with water, until the alcohol diffuses 

 through 2 or 3 mm. of the small pores of the pot. While 

 this diffusion is going on, the water in contact with the mem- 

 brane is passing into the cell under normal osmotic conditions. 

 This gives the temporary rise. This inflow of the water 

 between the outside alcohol and the membrane helps the 

 alcohol to reach the membrane in a shorter time than would 

 be required if the water within the pores were stationary. 

 When the alcohol is in contact with the membrane, the fall 

 in pressure begins. In support of this explanation there is 

 the absence of any temporary rise in the experiment of 

 Table III. 



This explanation also accounts for the temporary fall when 

 the cell is taken from the alcohol and is placed in water. 

 Now there is a layer of alcohol in the pot between the mem- 

 brane and the outside water. So long as this remains there 

 is an outflow. When the water reaches the membrane, the 

 permanent and normal rise of pressure begins. 



If this way of accounting for these temporary effects be 

 the true one, it follows that, if the cell remain in the water 

 until the pressure set up is at its maximum value an d be then 

 placed in alcohol, there will be no temporary rise. The 

 osmotic equilibrium will last so long as water is in contact with 

 the outer face of the membrane ; and further, since there is 

 now no inflow, the alcohol will have to diffuse through a layer 

 of water whose thickness is not reduced by any passage 



