454 C. Barus — Colloidal Silver. 



Oberbeck's* electric work, undertaken in a different direction 

 with a view of showing the occurrence of an allotropic state in 

 Carey Lea's silver has quite failed to convince us. We see no 

 reason for withdrawing the views which we originally expressed 

 (1. c.) 



3. At the close of our experiments f we suggested that a 

 Held of great promise would be found in the metallic optics of 

 colloidal silver. Work of this kind has since been taken up 

 with success by Wernicke.:}; The change of phase observed on 

 the reflection of light from thin metallic laminae enclosed 

 between clear media, with the front plate of greater refractive 

 index, is either an acceleration or a retardation, according as 

 the metal is intrinsically coherent or non-coherent. The 

 method is remarkably sensitive and applicable to metallic films 

 so thin as to be quite invisible to the eye. Tested in this way 

 colloidal silver according to Wernicke§ takes rank with bodies 

 which in their ultimate nature are an aggregate of individual 

 particles, however small they may be, or however perfect the 

 mirroring surface which an even distribution may produce. 

 It is this method which is to be looked to for decisive results, 

 not only for silver but for other colloids. 



4. Of the two interpretations which may be given of Carey 

 Lea's brilliant discovery, the one originally advocated by Dr. 

 Schneider and myself is to me intensely the more interesting. 

 As an aggregate of excessively fine suspended particles, colloidal 

 silver introduces a whole series of fascinating physical problems, 

 subject to forces which as to their nature are almost tangible. 

 Even in an ordinary case of sedimentation if I write 



Muddy water + acid = acidulated water + mud, 



the latter body being precipitated, I have a chemical equation 

 in embryo, — an equation! which so far as can now be discerned 

 lacks stochiometric precision, but which in its general charac- 

 ter is undoubtedly a double decomposition. If the actuating 

 forces be traced, they must lead by slow gradations up to 

 affinity. 



Washington, October, 1894. 



*Oberbeck: Wied. Ann., Xlvi, p. 265, 1892; xlvii, p. 353,1892; 

 f Barus u Schneider: Wied. Ann , xlviii, p. 336, 1893. 

 % Wernicke: Wied. Ann., li, p. 448, 1894; hi, p. 515, 1894. 

 § Wernicke : 1. c. p. 523. 



jjln the same way I picture to myself, the remarkable physical effects pro- 

 duced by traces of foreign admixtures in metallurgy. 



