M. Carey Lea — Allotropic Silver. 487 



that the substances with which tannin produces these reactions 

 would be further increased by investigation. 



I have found some additional modes of production of these 

 forms of silver, modes which are very curious and interesting. 

 They are now being studied and will be reported on hereafter. 



Nature of the " intermediate substance." 



It has been mentioned in previous papers that wheu allo- 

 tropic silver is converted into normal silver by the action of 

 heat it passes through a perfectly well marked intermediate 

 state. In this state it retains the gold-yellow color and high 

 luster but none of the other properties of the original form. 

 Oxidizing and chlorizing agents show nearly the same indif- 

 ference as with ordinary silver. While allotropic silver is soft 

 and easily reduced to powder the intermediate substance is 

 hard and tough. When a glass rod is drawn over a film of 

 allotropic silver it leaves behind it a white trace of ordinary 

 silver. The intermediate substance shows no such 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 color. 

 Continued exposure to sunlight brings about the same altera- 

 tion 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 crys- 

 talline form. Some films spread on paper were exposed to the 

 action of very dilute 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, while the portion that had passed into 

 the intermediate form retained its bright gold-yellow color'and 

 luster, rendering it thus distinguishable. The figures which it 

 exhibited were strikingly crystalline. One portion showed a 

 foliated structure such as is formed by interpenetrating crys- 

 tals, other parts showed ramifications with something of a 

 plant-like form. Another part exhibited a sheaf of acicular 

 crystals nearly parallel in direction, 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 spon- 

 taneous alteration to make the structure manifest. The altera- 

 tion is not apparent to the eye as the color does not change. 



This change to the crystalline condition does not seem to be 

 peculiar to gold-colored 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 



