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LXXVIIT. Periodic Precipitates. 

 By the late Lord Rayleigh, O.M., F.E.S. 



[Note. — This paper was found in my father's writing-table 

 drawer. It is not dated, but I believe it was written in 1917. 

 It was no doubt withheld in the hope of making additions- 

 It is published exactly as found. — Bayleigh.] 



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 Punge in 1885. Since 

 that time these precipitates have been studied by a number 

 of authors, and particularly by P. Liesegang of Diisseldorf, 

 who in 1907 published a work on the subject, entitled ' On 

 Stratification by Diffusion V 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 while 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. The 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 concentration, the less- 

 is the interval between the rings." 



In considering an explanation, the first question which 

 presents itself is why should the precipitate be intermittent 

 at all ? I suppose the answer is to be found in the difficulty- 

 of precipitation without a nucleus. At a place where the- 

 second material (silver nitrate) has only just penetrated, 

 there may be indeed a chemical interchange, but the 

 resultant (silver arsenate) still remains in a kind of solution.. 

 Only when further concentration has ensued, can a pre- 

 cipitate in the usual sense be formed, and a visible line 



* Translated by W. Deane Butcher, Rebrnan Limited, Shaftesbnry 

 Avenue, London. 



