M. Carey Lea — Properties of Allotropic Silver. 237 



again; this is the only occurrence of such irregularity. There- 

 fore giving it about one-third weight, we find as the most 

 probable value 



1 B. A. UNIT =*9863 OHMS. 



A determination of the " Mercury Unit " was recently made 

 by Messrs. Hutchinson and Wilkes (Johns Hopkins University 

 Circulars, May, 1889), who found the value to be 



•95341 



taking this with the above number for the B. A. unit, we have 

 as the length of the mercury column corresponding to the ohm, 



106-34 cms. 



Art. XXXI Y. — The Properties of Allotropic Silver ; by 

 M. Carey Lea. 



The three forms of allotropic silver which were described 

 in the June number of this Journal — the blue soluble and the 

 blue and the yellow insoluble — are not to be understood as the 

 only forms which exist, but as the best marked only. The 

 substance is protean, and exhibits other modifications not yet 

 studied. No other metal than silver appears to be capable of 

 assuming such a remarkable variety of appearances. Every 

 color is represented. I have obtained metallic silver blue, 

 green (many shades of both), red, yellow and purple. In 

 enumerating these colors I do not refer to interference colors 

 produced superficially by reagents, also wonderfully brilliant, 

 but to body colors. As a single instance of coloration the 

 following may be mentioned. I recently obtained a solution 

 of allotropic silver of an intense yellow brown. A little solu- 

 tion of disodic phosphate changed this to blight scarlet (like 

 Biberich scarlet) presently decolorizing with formation of a 

 purple precipitate. "Washed on a filter this changed to bluish 

 green. The colors I have met with in this investigation can 

 only be compared with the coal-tar products, of which one is 

 constantly reminded by their vividness and intense colorific 

 power. 



Two of the insoluble forms of allotropic silver, the gold- 

 colored and the blue, show in many respects a close relation- 

 ship and almost identical reactions. There are other respects 

 in which they differ strikingly and amongst these, in stability. 

 Blue allotropic silver (dark red whilst moist, becoming blue in 

 drying) is very stable. It may be exposed for weeks in a 



