248 Mr. M. Carev Lei 



a on 



It is evident that a connexion must almost certainly exist 

 between these varied transformations and the changes which 

 many silver salts undergo through the action of light and 

 other forces. This connexion will form the subject of the 

 second part of this paper. The present part will be concluded 

 by a somewhat fuller description of the colour reaction which 

 is especially characteristic of allotropic silver. 



The Colouk Reaction. 



When allotropic silver is immersed in a solution of a sub- 

 stance readily parting with oxygen or sulphur, or with a 

 halogen, a film is formed which exhibits the colours of thin 

 plates. Such phenomena are familiar, and are seen in the 

 bluing or yellowing of steel in tempering, and the colouring 

 of other metals when covered with films of oxide or sulphide. 

 With allotropic silver the colours are very brilliant, probably 

 because silver is the best of all reflectors for rays having a nearly 

 perpendicular incidence, sending back about 90 per cent, of 

 such. Light gold-coloured silver gives the most brilliant 

 effects. 



The substances which produce these reactions are potassium 

 ferricyanide and permanganate, ferric and mercuric chlorides, 

 alkaline hypochlorites and sulphides, mixtures of potassium 

 bichromate with hydrochloric or hydrobromic acid, solution 

 of iodine, &c. 



Potassium ferricyanide in a five or ten per cent, solution 

 is the best of these reagents, because its action is more dis- 

 stinctive. In particular, the blues which it gives are of great 

 purity and the purples very rich. Ferric chloride gives 

 beautiful tints, especially a peculiar glittering rose-colour. 

 It must be very much diluted, until the solution loses its 

 yellow colour and takes a straw shade. It often happens that 

 the characteristic colour does not appear whilst the film is 

 in the solution, but a bronze shade only, the permanent colour 

 appearing only after the film has been dipped into water and 

 blotted off. Potassium permanganate also gives rise to a 

 beautiful succession of colours on 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 



