Gold- coloured Allotropic Silver. 249 



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 grey 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, con- 

 verting the silver colour to gold. It is needless to say that 

 this gold colour has nothing to do with allotropic silver ; it 

 seems to be produced in the following way : — 



When potassium ferricyanide acts on films of allotropic 

 silver, its first effect is to deepen the gold colour 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 colours 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. 



Third Order. 

 Reddish purple. 

 Bluish purple. 

 Rich green. 



The fourth order is not reached, for after this the colours 

 become much mixed, probably the action is no longer suffi- 

 ciently uniform. The other differences, besides the absence 

 of the fourth order, as will have been observed, are that, in 

 place of the yellow of the second order corresponding to a 

 thickness of air of 0*000432 mm., there is a green though 

 of a more russet shade than that of the third order. 



In the third order there is at no time a pure blue 

 corresponding to 0*000602, but only a succession of beautiful 

 red and blue purples, gradually passing into green. 



