IX] OF CERTAIN RHYTHMIC PHENOMENA 667 



precipitation depends only on forces intrinsic to the system, and 

 is independent of any corresponding rhythmic changes in external 

 conditions, we have not far to seek for analogous chemico-physical 

 phenomena where rhythmic alternations of structure are produced 

 in close relation to periodic fluctuations of temperature. The 

 banding, or "varving," of Swedish and Irish glacial clays is a re- 

 markable instance. A well-known and a simple case is that of the 

 Stassfurt deposits, where the rock-salt alternates with thin layers of 

 "anhydrite," or (in another series of beds) with " polyhahte * " : 

 and where these zones are commonly regarded as marking years, 

 and their alternate bands as due to the seasons. A discussion, 

 however, of this remarkable and significant phenomenon, and of 

 how the chemist explains it, by help of the "phase-rule," in con- 

 nection with temperature conditions, would lead us far beyond our 

 scope. 



We may turn aside to touch, for a single moment, on certain 

 forms and patterns not easy to classify: some of which depend on 

 the molecular structure of a colloid matrix, while others are of a 

 coarser and more mechanical grade. So many organic forms and 

 patterns await explanation that we cannot seek too widely for 

 examples, nor for explanations, of such things. For instance, a 

 drop of dried egg-albumin shews beautiful radial cracks, with cross- 

 lines here and there; and a drop of blood drying on a glass plate 

 shews a complete system of radial fissures, in series after series, 

 sometimes with and sometimes without a clear central space. The 

 general resemblance to the cross-section of a stem, with its pith and 

 its primary and secondary medullary rays, is striking enough to 

 have led some even to look upon a tree as one great complicated 

 but symmetrical colloid massf. We may compare also the beautiful 

 radiating structure which Biitschli observed long ago around small 



* The anhydrite is sulphate of lime (Ca>S04) ; the polyhalite is a triple sulphate 

 of lime, magnesia and potash (2CaSO4.MgSO4.K2.SO4 + 2H2O). 



t Cf. H. WisHcenus, Ztschr. f. Chemie u. Kolloide, vi, 1910; A. Lingelsheim, 

 PHanzenanatomische Strukturbilder in trocknenden Kolloiden, Arch.f. Entw. Mech. 

 XLii, pp. 117-125, 1917. Cf. also Liesegang, Trocknungserscheinungen bei Gelen, 

 Ztschr. f. Ch. u. K. x, p. 229 sq., 1912; Biitschli, Verh. n. h. Ver. Heidelberg, vii, 

 p. 653, 1904. Also {int. al.) Norman Stuart, on Spiral growths in silica gel, Nature, 

 Oct. 2, 1937, p. .589. 



