CHATTER XI 



OSMOTIC GROWTH— A STUDY IN MORPHOGENESIS 



The phenomenon of osmotic growth has doubtless presented 

 itself to the eves of every chemist ; but to disc-over a pheno- 

 menon it is not enough merely to have it under our eyes. 

 Before Newton many a mathematician had seen a spectrum, 

 if only in the rainbow; main' an observer before Franklin 

 had watched the lightning. To discover a phenomenon 

 is to understand it, to give it its due interpretation, and to 

 comprehend the importance of the role which it plays in the 

 scheme of nature. 



Osmotic Membranes. — Certain substances in concentrated 

 .solution have the property of forming osmotic membranes 

 when they come in contact with other chemical solutions. 

 When a soluble substance in concentrated solution is immersed 

 in a liquid which forms with it a colloidal precipitate, its 

 surface becomes encased in a thin layer of precipitate which 

 gradually forms an osmotic membrane round it. 



An osmotic membrane is not a semi-permeable membrane, 

 as sometimes described, i.e. a membrane permeable to water 

 but impermeable to the solute. It is a membrane which 

 opposes different resistances to the passage of water and of 

 tile various substances in solution, being very permeable to 

 water, but much less so to the different solutes. 



A soluble substance thus surrounded by an osmotic 

 membrane represents what Traube lias called an artificial 

 cell. In such a cell the dissolved substances have a very 

 high osmotic pressure, an expansive force like that of 

 steam in a boiler; the molecules of the solute exerting pressure 

 on the walls of the extensible cell, and distending it like the 



