Section IV, 1917 [117] Trans. R.S.C. 



Concentric Ridges on Naturally Occurring Silica 



By J. Stansfield, B.A., F.G.S. 

 Presented by Dr. Frank D. Adams, F. R.S.C. 



(Read May Meeting, 1917). 



In some experiments carried out by the writer during an investiga- 

 tion of the phenomenon known under the name of Liesegang rings,^ 

 in which gelatine containing different reagents of various strengths 

 placed upon glass plates was used, banded structures were occasionally 

 met with in the gelatine as a result of drying. The structures referred 

 to consist of concentric ridges on the dried surface of the gelatine, 

 whose courses are evidently controlled by the outline of the sheet of 

 gelatine on the plate, being roughly parallel, at first, to the outer 

 contour of the gelatine. 



These ridges are not always formed in the drying of gelatine, in 

 fact, they have been noticed in only a few cases. The best examples 

 are those in which the gelatine contained a N/20 solution of lead 

 nitrate. So that it is difficult to determine exactly the underlying 

 causes of their formation. Liesegang has noted that in the drying of 

 gelatine containing sodium phosphate the phosphate was deposited in 

 crystals arranged in banded layers.^ The writer has noticed a similar 

 banding of crystals in the drying of gelatine containing a strong 

 solution of potassium chromate. Thus it is clear that the drying of 

 gelatine is accompanied by diffusion and variation of concentration 

 of reagents in it. No solution of the problem of the formation of 

 ridges in drying is attempted here, but the suggestion is offered that 

 variation of surface tension as a result of diffusion and attendant 

 changes of concentration may have been an important factor in 

 producing the ridges. 



The point of interest to the writer in these ridges is that they have 

 been formed artificially under certain conditions as a result of the dry- 

 ing of gelatine, and that they are not always formed when these condi- 

 tions seem to be fulfilled, and that very similar structures have been 

 developed naturally in colloidal silica. The first example which came 

 under the writer's notice was a specimen of Streptelasma profundum 



1 Stansfield, J. Amer. Jour. Scl., XLIII, 1917, p. 1. 



- Liesegang, R. E. Geologische Diffusionen, 1913, p. 92, 



