XLII. Notes on Allotropic Silver. By M. Carey Lea*. 



nELATIONS of the Yellow to the Blue Forms.— The gold 

 -**' and copper-colon red forms on the one hand, and the 

 blue, bluish-green, and steel-grey on the other hand, stand 

 in close relations to each other. In previous papers there has 

 been described a crystalline state intermediate between these 

 active forms and ordinary silver, which intermediate condition, 

 whilst retaining the bright yellow colour of the active form, is 

 nearly as indifferent to reagents as ordinary silver. Into this 

 intermediate state both the yellow and blue forms are capable 

 of passing, and apparently the intermediate states of both 

 kinds of allotropic silver are identical : the intermediate form 

 of blue silver is yellow. Thus, when lumps of blue silver are 

 heated in a test-tube to about 180° C, they assume a gold 

 colour and lustre. The same change takes place at the same 

 temperature when films of blue silver are placed in a hot-air 

 bath. 



But relations much closer than these exist. Blue silver can 

 be converted into yellow at ordinary temperatures and con- 

 sequently with retention of its active properties. This is 

 accomplished through the agency of sulphuric acid. When a 

 solution of silver is obtained by the action of sodium hydroxide 

 and dextrine on silver nitrate f, it appears to contain the blue 

 variety ; for if allowed to precipitate spontaneously by long- 

 standing, or if precipitated by acetic acid, dilute nitric acid, 

 or by many neutral substances, it gives a form of silver which 

 is dark red whilst moist and dries with a blue surface-colour. 

 (It is always a little difficult to characterize these substances by 

 their colours, since the surface-colour which they show when 

 dry, either in mass or in films, is mostly complementary to 

 their colour when wet. As the surface -colour is much the 

 more characteristic^ I have adopted the course of naming 

 them by that.) 



The behaviour of the red solution obtained by soda and 

 dextrine with dilute sulphuric acid is very interesting and 



* From an adyance proof communicated by the Author. 



| Forty grams each of sodium hydroxide and of yellow or brown dex- 

 trine (not white) are dissolved in two litres of water, and twetny-eight 

 grams of silver nitrate in solution are added in small quantities at a time, 

 with frequent stirring, so that several hours shall elapse before the last 

 portion is added. The solution is always slightly turbid when viewed by 

 reflected light, by which it shows a beautiful deep green colour. By trans- 

 mitted light it is deep red, and, when diluted, absolutely transparent. 

 By diminishing the proportion of silver nitrate to one half, a solution 

 nearly or quite clear by reflected as well as by transmitted light is obtaiDed. 



Phil. Mag. S. 5. Vol. 32 No. 197. Oct. 1891. 2 A 



