OSMOSIS 185 



potassium f errocyanide (K4Fe(CN) e) . A precipitation membrane 

 of copper f errocyanide (Cu2Fe(CN)6) is immediately formed 

 around the sulphate crystals. Water enters, but the sulphate 

 molecules cannot get out and establish an equilibrium in concen- 

 tration; consequently, the membrane is subjected to a pressure 

 within until it bursts. This immediately exposes a new surface 

 between the inner copper sulphate and the outer potassium ferro- 

 cyanide, which results in a patch being formed by a new 

 precipitate of copper ferrocyanide. Again water enters in excess, 

 turgor is produced, the membrane bursts and is repaired once 



Fig. 101. — "Growing" osmotic systems with copper ferrocyanide membranes. 



more. In this way, the miniature osmotic system grows (Fig. 

 101). It has been suggested that growth in organisms is of this 

 sort. Some similarity there may be, but growth involves more 

 than mere distention. 



A more refined and far more secure type of osmometer is made 

 by supporting the Traube precipitation membrane within the 

 walls of a porous clay cup. This is done by filling the cup with 

 copper sulphate and immersing it in potassium ferrocyanide. 

 The precipitation takes place where the two salts meet within 

 the wall of the cup (Fig. 102). Such a supported copper ferro- 

 cyanide membrane is not so readily broken. It may therefore 

 withstand great pressures and for this reason has been much 

 used for experimental work. 



A copper ferrocyanide membrane has certain properties which 

 are very similar to those of the living cell membrane; thus, it is 

 impermeable to certain salts (copper sulphate and potassium 



