290 Dandeno : Osmotic theories 



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ever, to this discussion — is that the membrane is weaker at the 

 top, owing to the fact that the copper chloride is less dense there, 

 and the consequently less rapid chemical action ; and also to the 

 fact that the membrane is not so old at, or near, the top. The 

 phenomena which may be observed in the formation of the Traube 

 pellicle present a strong argument against the "gas-pressure*' 



theory. 



Take a glass jar and place in it a bright, well-cleaned piece 

 of zinc, or bright iron, then fill with distilled water, and cover 

 tightly with a bladder parchment as for an osmometer. Now im- 

 merse the whole jar in a strong solution of copper sulphate. After 

 a time the zinc (or Iron) becomes coated over with a dark brown 

 substance which proves to be copper, or copper oxide. If a similar 

 experiment be made, but without the zinc strip, there is no copper 

 sulphate found in the closed jar, even after a few hours. Now the 

 only reasonable conclusion as to how the copper molecules, or 

 ions, reached the zinc, is that they were attracted there by the zinc 

 and not driven from behind. 



The matter of interpretation, or explanation of the gas-theory 

 as accounting for osmotic pressure, shows perhaps as forcibly as 

 the experiment mentioned, that the theory of gas-pressure is 

 wholly inadequate to the situation. Two or three illustrations 

 of the conclusions reached by those basing their explanations on 

 the *' gas-pressure " theory may be given to demonstrate what 

 such theory leads to. Take, for example, the explanation 

 often given of osmotic pressure. Livingston * states it thus : 



*'The following explanation of osmotic pressure has been given 

 by various authors. The quotation is from Davenport [Experi- 

 mental Morphology]: *Upon the side containing the greater num- 

 ber of molecules of salt [solute], fewer water [solvent] molecules 

 will in a given time strike the membrane than upon the other side ; 

 and since the number passing through is proportional to the number 

 striking, relatively fewer molecules of water will consequently pass 

 out, and so there will be a resultant flow of water to that side ; and 

 if the mass of water is confined, it will exert great pressure.' This 

 explanation is untenable for several reasons. . . . Into this matter 

 it is unnecessary to go farther than to add that osmotic pressure 

 can be demonstrated as readily in solutions occupying less volume 

 than the original solvent as in those occupying more. It is obvious 



*The role of diffusion and osmotic pressure in plants. 



