262 



COLLOIDS IN BIOLOGY AND MEDICINE 



experiment may be performed by solidifying the ammonium bichro- 

 mate gelatin in a test tube and layering some silver nitrate over it. 

 We thus get instead of rings true precipitation membranes, which 

 are separated from one another by layers containing no silver chro- 

 mate (see Fig. 47). Subsequently, WILHELM OsxwALD,* 2 J. HAUS- 

 MANN,* H. W. MORSE and G. W. PIERCE,* H. BECHHOLD* 2 and 

 E. HATSCHEK studied the development of these rings. It may be 

 assumed as a result of these investigations that the formation of 

 such layers is the result of a very complicated combination of events 

 whose further elucidation at this point would 

 carry us too far afield. 



It should be definitely stated that the de- 

 velopment of rhythmic structures is in no way 

 dependent on the interdiffusion of two solutions. 

 Similar structures may be produced in jellies also 

 by crystallization (e.g., tri-sodium phosphate) or 

 by freezing water. 



The ring formation occurs especially when 

 ammonium chromate and silver nitrate come in 

 contact; at times there may be produced as 

 many as twenty or more parallel membranes, 

 which, according to the concentration of the 

 solution, may be separated from a fraction of a 

 millimeter up to 1/2 centimeter. They have also 

 been produced by numerous other precipitation 

 reactions. 



H. BECHHOLD prepared similar stratified mem- 

 branes with organic material. This occurs readily 

 if serum mixed with gelatin is permitted to solidify 

 in a test tube and metaphosphoric acid is layered 

 over it. The number and beauty of the mem- 

 branes depend very much on the relative con- 

 centrations of the solutions employed. Most 

 advantageous is a mixture of 2.5 per cent serum 

 and of 5 per cent gelatin, upon which is placed 

 2 per cent metaphosphoric acid; this gives as many as five concentric 

 rings. The author obtained two parallel membranes by the diffusion 

 of goat serum into gelatin, containing goat-rabbit serum. 



It is evident in the formation of this kind of layered membranes 

 that the phenomenon noted on page 85 et seq. plays an important role: 

 colloids only precipitate in definite mixture relations, and solution oc- 

 curs in the presence of an excess of either one. This phenomenon can 

 be followed visually in the above-described membrane formation. 



FIG. 47. Stratifica- 

 tions in a test tube. 

 (F. Stoffel.) 



