I2S 



SCIENCE PROGRESS 



rin^. Leisegang placed a drop of strong silver nitrate solution 

 on a glass plate coated with gelatin containing a trace of 

 potassium dichromate. As the silver salt diffused into the 

 gelatin the insoluble silver dichromate was precipitated in rings 

 surrounding the drop. The same thing happens when the 

 reagent is poured on top of the gel contained in a test-tube as 

 sho\\Ti in the figure. Bands are produced with many pairs of 

 reagents, in some cases with very beautiful results. 

 The distance apart of the rings or strata varies 

 in different cases from a fraction of a millimetre 

 to a centimetre or more. A good deal of attention 

 has been given to the subject, from which it 

 appears that the effect is due, in the main, to 

 exhaustion of the soluble reaction component from 

 the layer of gel adjacent to each band. The 

 banding occurs only when the precipitate has large 

 specific surface. The reagent is adsorbed from the 

 gel by the molecular forces residing in the surface 

 of the precipitate. When all the reagent has been 

 extracted from the region of gel next to the 

 precipitate, no more of the latter can be formed, 

 and the precipitating reagent, diffusing into the 

 gel, passes through the exhausted zone to a region 

 where more of the dissolved salt is available for 

 the formation of a fresh band. This banded 

 structure is found in a great many different varieties 

 of minerals. Liesegang showed that the rings of 

 agates are formed in this way. Recently it has 



ebeen suggested that the bands of flint in chalk 

 are another case. Probably many points, hitherto 

 obscure, in connection with the origin of mineral 

 deposits are capable of similar explanation. In 

 biology the phenomenon may prove to be of much 

 importance. Various tissues have this lined aspect. 

 This is the structure of muscle. In many cases 

 the WTngs of insects and the plumage of birds 

 show the same banded appearance. To their 

 striated surface must be ascribed some of the most 

 beautiful colours of nature : the fire of the scarab beetle, 

 the colours of the peacock, and the iridescence of the 

 pearl. 



Of the two problems of transformation of supreme con- 

 sequence to biologists, the artificial metamorphosis of one 

 species of animals or plants into another may be said to have 

 been solved, as is evinced by the continual introduction of 

 improved varieties of flowers and vegetables. The other 

 problem, the artificial transmutation of dead into living matter, 



SilTer chro- 

 mate bands 

 in agar jelly. 



