124 American Fisheries Society 



are living. Ideal semi-permeability with respect to particular 

 dissolved substances has been achieved, and is found in living 

 organisms. 



It is to be remembered that in case of semi-permeable 

 membranes, the solvent will flow from the less concentrated 

 to the more concentrated side of the membrane, so that if we 

 wish to extract water we need only to make the outside more 

 concentrated than the inside, if we wish to add water we make 

 the outside less concentrated than the inside, i. e., we use pure 

 fish, and that permeability increases at temperatures near the 

 freezing point of water. 



It is also to be remembered that membranes do not neces- 

 sarily hold their degree of semi-permeability unalterable; the 

 permeability of the membrane can very readily be changed, 

 as will be seen later. There is reason for believing, for ex- 

 ample, that the permeability of fish to salt increases after 

 death, for stale fish strike through more quickly than fresh 

 fish, and that permeability increases at temperatures near the 

 freezing point of water. 



The tissues of fish consist of cells; each cell is a bag of 

 semi-liquid, like the white of egg. The surface of every cell 

 either is, or acts like, a semi-permeable membrane. If we sur- 

 round the cell with water, the inside will be more concentrated 

 than the outside, and water will go in; if we surround the cell 

 with strong salt solution, water will pass out to the salt. Some 

 salt will also pass into the cell, which fact shows that the cell 

 wall is not ideally semi-permeable. 



But what of the protein within the cell ? Why does it not 

 come out while the salt is going in ? The answer to these ques- 

 tions makes it necessary to pass from a consideration of the 

 nature of the membrane in osmosis to a consideration of the 

 nature of the dissolved substance. 



By a great many experiments it has been found that some 

 dissolved substances never pass through membranes under any 

 circumstances, while others will pass through some membranes. 



