[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. Since that time these precipitates have been studied by a number of authors, and particularly by R. Liesegang of Düsseldorf, 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.