M. C.Lea — Gold-colored Allotropic Silver. 189 



In particular the blues which it gives are of great purity and 

 the purples very rich. Ferric chloride gives beautiful tints, es- 

 pecially a peculiar glittering rose color. It must be very much 

 diluted, until the solution loses its yellow color and takes a 

 straw shade. It often happens that the characteristic color 

 does not appear whilst the film is in the solution but a bronze 

 shade only, the permanent color appearing only after the film 

 has been dipped into water and blotted off. Potassium per- 

 manganate also gives rise to a beautiful succession of colors 

 ou allotropic silver but is somewhat uncertain in its action. 



A ferricyanide is therefore the best reagent. As to the 

 substance constituting the film which is formed, it is difficult 

 to say whether it is silver suboxide or ferrocyanide. When 

 potassium ferricyanide is allowed to act on moist allotropic 

 silver suspended in it, and the action of the ferricyanide is 

 carried to its limit, the silver is entirely converted into a 

 yellowish white powder, consisting almost wholly of silver 

 ferrocyanide mixed with a little silver ferricyanide. 



Of the many varieties of ordinary silver which exist allo- 

 tropic silver is convertible into two only. The high tension 

 spark, heat, and acids convert it to dull gray silver : on this 

 variety potassium ferricyanide has no action whatever, as will 

 be seen by an inspection of the plates. Light under glass and 

 pressure each convert allotropic to bright white silver, and on 

 this form potassium ferricyanide acts slightly, converting the 

 silver color to gold. It is needless to say that this gold color 

 has nothing to do with allotropic silver : it seems to be pro- 

 duced in the following way. 



When potassium ferricyanide acts on films of allotropic 

 silver, its first effect is to deepen the gold color to a gold 

 brown, passing rapidly on to other shades. The action on the 

 bright white silver is very slight and apparently just reaches 

 this gold stage, which corresponds to an air film having a 

 thickness of from-0*000150 mm to 0'000160 mm . 



The succession of colors obtained on allotropic silver with 

 potassium ferricyanide is as follows : 



First Order. 



Russet brown. 

 Brown red. 



Second Order. 



Rich and deep purple. 



Dark blue. 



Bright blue. 



Pale blue. 



Green russet. 



Red. 



