Gold-coloured Alio tropic Silver. 245 



white silver formed had the fine frosted white colour of pure 

 silver. This change was attributed to the friction of the frag- 

 ments against each other, occasioned by the motion during 

 the journey; a conclusion that was confirmed by finding 

 that when cotton-wool was forced into the empty part of the 

 vial in such a way as to prevent all internal movement, the 

 substance could be sent over a four-fold distance without 

 alteration. 



It was also observed (and this is a matter of special 

 interest) that when a partial change had been effected by 

 friction, this alteration went on, although the substance was 

 left perfectly at rest, until it became complete. With time, 

 all solid specimens of allotropic silver undergo this sponta- 

 neous change to bright white silver, apparently normal silver, 

 even when thoroughly protected from light. Out of over 

 twenty specimens in tightly corked tubes, packed in a box 

 and left in a dark closet for a year, not one escaped conver- 

 sion. Spread on paper or on glass and duly protected, the 

 change is slower. 



i &' 



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 

 gaseous matter ; not a bubble can be detected by the closest 

 observation. 



By acting on dry films with dilute sulphuric acid it is eas} r 

 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 

 intermediate form. It is then bright gold yellow, but shows 

 no colour with the ferricyanide reagent. 



With sulphuric acid diluted with twice its bulk of water 

 and used whilst still hot, the action is instantaneous, and the 

 allotropic silver is converted into light grey, normal silver. 

 The silver obtained in this way is very indifferent and gives 

 no reaction with potassium ferricyanide, whereas even ordinary 

 silver leaf gives a pale-coloured reaction. (See Plate II., lower 

 pair of figures.) 



The same acid after cooling acts more slowly ; the product 

 is more yellowish, owing to the presence of a certain quantity 

 of the intermediate form. 



Phil. Mag. S. 5. Vol. 31. No. 190. Mar. 1891. T 



