Gold-coloured Mlotropic Silver. 241 



cates some remarkable molecular change. It will be noticed 

 that the anomaly lies in this, that pressure instantly effects the 

 complete change from the original form to normal silver, heat 

 effects the same change but with an intermediate stage, at 

 which stage 'pressure no longer produces any action. 



The intermediate form is distinguished from normal silver 

 almost solely by its bright yellow colour and its higher lustre. 

 This last difference is very striking when a film on glass is 

 heated in the manner above described. The central parts, in 

 changing to white silver, become wholly lustreless ; whilst the 

 circle of " intermediate " retains all its original lustre. Its 

 continuity is still complete, so that if viewed through the 

 glass it still acts as a mirror. 



This change may be either molecular or depend on 

 dehydration. 



The latter seems doubtful, for the change cannot be brought 

 about by desiccation. Films on paper, on glass, and also 

 solid material were kept over sulphuric acid in vacuo for 

 twelve days * without bringing about this modification. (They 

 were of course thoroughly protected from light.) 



Light is also capable of effecting to some extent this 

 change, as will be described further on. 



COPPER-COLOURED ALLOTROPIC SlLVER. 



The colour of ailotropic silver depends to a remarkable 

 extent on the amount of washing which the freshly prepared 

 material receives. 



With a short washing the material dries to a bright yellow- 

 gold colour ; with more washing to a reddish colour ; with 

 still more, the colour is a deep rich copper shade. 



The washing, when conducted in the ordinary manner, is 

 exceedingly troublesome ; the material soon begins to run 

 through the filter, and blocks it up. 



This trouble may be completely avoided by washing with a 

 2-per-cent. solution of Rochelle salt instead of pure water, 

 until towards the end of the operation! . 



* A longer time was inadmissible on account of the tendency to spon- 

 taneous alteration. 



f The mode of preparing the gold- and copper-coloured forms is as 

 follows ; the difference is in the length of washing only. 



In a precipitating jar are placed : — 



Water 800 c.c. 



20-per-cent. sol. Kochelle salt 200 „ 



40-per-cent. sol. silver nitrate 50 „ 



