J. C. Blake — Colors of Allotropic Silver. 285 



matter or typical colloids. This effect of typical colloids in 

 rendering colloidal solutions of the metals more stable, dis- 

 covered by Faraday, has been investigated at length by Zsig- 

 mondy^ and others. 



Mirrors formed by spreading on glass portions of a precipi- 

 tate of ''red silver " containing electrolytes, like that obtained 

 by the action of ferrous citrate, transmit red light and reflect 

 indigo-blue light only while moist. At the moment of drying, 

 the substance suddenly changes over to '' blue silver " — a 

 change which undoubtedly served to prevent Lea from recog- 

 nizing clearly the distinction between " red silver " and " blue 

 silver,- ' especially as he was accustomed to observe the " sur- 

 face" and the "body" colors of these substances, instead of 

 differentiating the colors of the reflected and transmitted 

 light. When mirrors which show the color change are viewed 

 in ordinary light aftei* drying, the reflected (golden) and trans- 

 mitted (blue) lights are commingled and the mirrors have a 

 beautiful bright fluorescent green appearance. This effect was 

 seen by Lea, who determined that the blue and the yellow 

 colors were oppositely polarized. 



Another variety of green effect, noted by Gutbierf in the 

 action of hydrazine hydrate on a solution of silver nitrate, is 

 still more deceptive. The green in this case is due to an inter- 

 mixture of " blue silver " with " yellow silver " formed simul- 

 taneously. Li order to see this it is only necessary to act with 

 hydrazine hydrate on a solution of silver nitrate at considerable 

 dilution. No action takes place for some minntes ; then an 

 orange-yellow color appears in the liquid and a yellow mirror, 

 forms on the sides of the containiug vessel. Such yellow 

 mirrors may even form in the more concentrated green solutions. 

 In solutions still more concentrated pure blue effects may be 

 readily obtained. The same sequence of colors was observed 

 less clearly in other cases. 



The detailed observations are collected in the following 

 table. It is to be understood that the solutions used, both of 

 the silver compounds and of the reducing mixtures, were in 

 no case very strong, those marked '' concentrated " containing 

 about Ave or ten grams of the material per liter, those marked 

 " dilute " containing from a tenth to a hundredth part as much. 

 Care was taken to avoid a local excess of any electrolytic 

 reagents employed, and the reducing mixture was always finally 

 added in excess of the silver compound. Except for the pur- 

 pose of avoiding knowm precipitates, the order in which the 



* Zeitsclar. anal. Chem. xl, 697. Schulz and Zsigmondy, Hof meister's Bei- 

 trage, iii, 137. Lettermorer and Meyer, Jour, prakt. Chem. Ivi, 248. 

 f Zeitschr. anorg. Chem. xxxi, 448 ; xxxvii, 847. 



Am. Jour. Scl— Fourth Series, Yol. XYI, No. 94.— October, 1903. 

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