445. PERIODIC PRECIPITATES. [Philosophical Magazine, Vol. xxxvni. pp. 738—740, 1919.] [Note.—This paper was found in the author's writing-table drawer after his death. It is not dated, but was probably written in 1917. It was no doubt withheld in the hope of making additions.] I OWE my knowledge of this subject, as well as beautiful specimens, to Prof. S. Leduc of Nantes. His work on the Mechanism of Life* gives an account of the history of the discovery and a fairly detailed description of the modus operandi. "According to Prof. Quincke of Heidelberg, the first mention of the periodic formation of chemical precipitates must be attributed to Runge in 1885 f. . Since that time these precipitates have been .studied by a number of authors, and particularly by R-. Liesegang of Dusseldorf, who in 1907 published a work on the subject, entitled On Stratification by Diffusion" In 1901 and again in 1907 Leduc exhibited preparations showing concentric rings, alternately transparent and opaque, obtained by diffusion of various solutions in a layer of gelatine. "The following is the best method of demonstrating the phenomenon. A glass lantern slide is carefully cleaned and placed absolutely level. We then take 5 c.c. of a 10 per cent, solution of gelatine and add to it one drop of a concentrated solution of sodium arsenate. This is poured over the glass plate whilst hot, and as soon as it is quite set, but before it can dry, we allow a drop of silver nitrate solution containing a trace of nitric acid to fall on it from a pipette. Tlfe drop slowly spreads in the gelatine, and we thus obtain magnificent rings of periodic precipitates of arsenate of silver... .The distance between the rings depends on the concentration of the diffusing solution. The greater the fallj of concentration, the less is the interval between the rings." * Translated by W. Deane Butcher, Rebman Limited, Shaffcesbury Avenue, London. [f The year " 1885" agrees with the source of the quotation (1. c. p. 67), but it appears that the correct date is " 1855." For in the original chronologically arranged historical passage in Prof. Quincke's paper " ijber mmchtbare Fliissigkeitsschichten u.s.w." in Annalen der Physik, Yierte Folge, Band 7 (1902), pp. 643—647, the first mention of the subject is attributed to F. F. Eunge in 1855; in agreement wibh the translation of this passage given at length in subsequent pages of The Mechanism of Life (see p. 118). $ The words "fall of" had been omitted from the quotation as printed in the original publication in the Philosophical Magazine. With this omission, the statement appears still to be valid, and to express more simply the fundamental property of the phenomenon. W. F. S.] 42—2ubt the existence of apparently trustworthy reports of many occult phenomena. For this there must be a reason, and our object is to find it. But whatever it may be, whether reality of the phenomena, or the stupidity or carelessness or worse of the narrators, a larger sweep is sure to add to the material. However, we may hope that such additions will occasionally affordation, one might expect husbands and wives with their heads within two or three feet of one another to share their dreams habitually. But there +-5, where /u, is the cosine of the angle between the secondary (or scattered) ray and the backward direction of the incident ray. W. F. S.]spheres are easily demonstrated.