502 Mr. M. Carey Lea on 



reaction : the trace of a glass rod does not differ from the 

 rest of the film ; and even hard burnishing produces no change 

 in the colour. Continued exposure to sunlight brings about 

 the same alteration to the intermediate form, and it takes 

 place spontaneously with time. 



At that time no explanation could be found as to the nature 

 of the change. It proves, however, to be a passage into a 

 crystalline form. Some films spread on paper were exposed 

 to the action of very dilate solution of ferric chloride. It 

 chanced that one of these films had undergone a partial 

 change into the intermediate form ; the unchanged portion 

 was darkened by the ferric solution, whilst the portion that 

 had passed into the intermediate form retained its bright gold- 

 yellow colour and lustre, rendering it thus distinguishable. 

 The figures which it exhibited were strikingly crystalline. 

 One portion showed a foliated structure, such as is formed 

 by interpenetrating crystals ; other parts showed ramifica- 

 tions with something of a plant-like form. Another part 

 exhibited a sheaf of acicular crystals nearly parallel in direc- 

 tion half an inch to an inch long, and as fine as hairs. These 

 appearances indicated with certainty crystalline structure. 

 Other specimens have been obtained, though none so well 

 defined as that just mentioned, which happened to be taken 

 at exactly the right stage of spontaneous alteration to make 

 the structure manifest. The alteration is not apparent to the 

 eye, as the colour does not change. 



This change to the crystalline condition does not seem 

 to be peculiar to gold -coloured silver. The blue form, when 

 gently heated in a tube, becomes yellow. By continued heat 

 it changes to white normal silver. A film on glass began to 

 change from blue to yellow at about 180° C. Light also pro- 

 duces this change on blue silver. The specimens obtained by 

 different processes act very differently ; some change with a 

 few hours of strong sunlight, others require many days. 



From what has been written in this and preceding papers, 

 it appears that allotropic and even soluble silver may be 

 formed in a great variety of reactions. The reducing agent 

 may be either a ferrous or a stannous salt, or any one of a 

 variety of organic substances of very different constitutions. 

 From the solubility and activity of this substance and the 

 parallelism which many of its reactions show to those of silver 

 in combination, I have been disposed to think that silver in 

 solution might, like silver in combination, exist in the atomic 

 form. It is certain that, up to the present time, we have no 



