186 M. 0. Lea — Gold-colored Allotropic Silver. 



escaped conversion. Spread on paper or on glass and duly pro- 

 tected, the change is slower. 



4. Action of Strong Acids. 



The action of acids upon allotropic silver has been already 

 described ; it remains only to add here that the conversion to 

 normal silver is entirely unaccompanied by the escape of gase- 

 ous matter ; not a bubble can be detected by the closest obser- 

 vation. 



By acting on dry films with dilute sulphuric acid it is easy 

 to make the conversion gradual and so to trace its passage 

 through the intermediate form. 



With sulphuric acid diluted with four times its bulk of 

 water and allowed to cool, an immersion of one or two seconds 

 converts a film on glass or on pure paper wholly to the inter- 

 mediate form.. It is then bright gold-yellow but shows no 

 color with the ferricyanide reagent. With sulphuric acid 

 diluted with twice its bulk of water and used while still hot, 

 the action is instantaneous and the allotropic silver is converted 

 into light gray normal silver. The silver obtained in this way 

 is very indifferent and gives no reaction with potassium ferri- 

 cyanide, whereas even ordinary silver leaf gives a pale-colored 

 reaction. (See Plate II, lower pair of figures.) The same 

 acid after cooling acts more slowly ; the product is more yel- 

 lowish, owing to the presence of a certain quantity of the 

 intermediate form. 



5. Action of Light. 



When allotropic silver is spread as a thin film on glass or on 

 pure paper it may be preserved for a length of time apparently 

 unchanged. This appearance is deceptive. From the moment 

 that the film is formed a slow but steady change commences 

 which can be best explained by supposing that a gradual poly- 

 merization takes place. Even after eight or ten hours' ex- 

 posure to ordinary diffuse light a distinct loss of activity can 

 be detected by careful testing with potassium ferricyanide. 

 The change which occurs is in the nature of a tendency to a 

 very gradual passage into what I have called the intermediate 

 form in which the gold-yellow color remains unchanged while 

 the chemical activity is lost or much diminished. Although a 

 commencement of this change can be detected in a few hours, 

 it goes on very slowly. By exposure to one or two days of 

 summer sunshine (a much longer time is required in winter), 

 the change is nearly complete. The exposed portions are 

 lighter and brighter, and in solution of ferricyanide they color 

 very slowly. 



