260 COLLOIDS IN BIOLOGY AND MEDICINE 



the interface against air it may be created by desiccation. Within 

 the organism, we must assume that membranes develop similar to 

 chemical precipitation-membranes. If we add silver nitrate to a 

 solution of common salt, a precipitate of silver chlorid is formed. 

 If we permit common salt and silver nitrate to diffuse together in a 

 jelly, at the point of contact a membrane of AgCl develops. We con- 

 sidered the results of this more thoroughly in the introduction to 

 this chapter. It is now, therefore, merely necessary to recall that not 

 only may crystalloids form such membranes in a jelly, but that with 

 albuminous material H. BECHHOLD* 2 produced such membranes in jel- 

 lies (phosphoric acid, goat serum and goat-rabbit serum). Theoreti- 

 cally, therefore, the development of membranes offers no difficulties. 



The development of a precipitate in a jelly gives a certain direction 

 to the further evolution of the process. The sense in which this is 

 intended will be elucidated by some examples; by membrane forma- 

 tion, to begin with. Substances which form no membrane by precipi- 

 tation, diffuse together unhindered, and in time become completely 

 mixed. If a semipermeable membrane has formed, it behaves like a 

 solid wall that arrests any further mixture. If two solutions of equal 

 osmotic pressure diffuse together in a jelly till they form a permeable 

 membrane, no matter how thin, e.g., sodium chlorid and silver nitrate, 

 diffusion ceases as soon as the membrane has a very slight thickness. 

 If, however, the osmotic pressure on one side is greater, the mem- 

 brane continues to grow until the osmotic pressure is equal on both 

 sides (N. PRINGSHEIM,* H. BECHHOLD and J. ZiEGLER* 1 ). 



The phenomena are exceedingly interesting when precipitates de- 

 velop simultaneously in several places. These phenomena have been 

 studied by R. LIESEGANG.* S If we place on a plate which is covered 

 with sodium chlorid jelly, a drop of silver nitrate, there forms a 

 disc-shaped precipitate of silver chlorid, whose circumference in- 

 creases equally in all directions (a circle) according as the silver 

 nitrate diffuses into the sodium chlorid jelly. If, however, two 

 drops are placed on the sodium chlorid jelly several centimeters 

 apart, there develops a picture like Fig. 46; the two silver chlorid 

 precipitates grow towards each other, that is, an " apparent chemical 

 attraction" is observed. The reason for this is as follows: im- 

 mediately upon applying the silver nitrate, the jelly loses sodium 

 chlorid because of the precipitation of AgCl, and this causes a 

 movement in the entire mass of sodium chlorid: the spot where the 

 precipitate forms is deprived of chlorin ions, which then diffuse 

 in afresh from the periphery. If two neighboring drops of silver 

 nitrate have been placed on the sodium chlorid jelly, there forms be- 

 tween them a region poor in chlorin, which thus permits a more rapid 



