182 M. C. Lea — Gold-colored Allotropic Silver. 



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 color and its higher luster. 

 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 lusterless, while the 

 circle of " intermediate " retains all its original luster. 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 dehy- 

 dration. 



The latter seems doubtful for the change can not 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 farther on. 



CoPPEE-CoLOEED ALLOTEOPIC SlLVEE. 



The color of allotropic 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 color ; with more washing to a reddish color ; with 

 still more, the color is a deep rich copper shade. The wash- 

 ing, 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 two per cent solution of Rochelle salt instead 

 of pure water, until towards the end of the operation.f 



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

 alteration. 



f The mode of preparing the gold and copper colored forms is as follows, the 

 difference is in the length of washing only. 

 In a precipitating jar are placed. 



Water 800 cc 



20# sol. Rochelle salt 2.00 



40^ sol. silver nitrate 50 



In another vessel are placed, 



Water 800" 



20$ sol. Rochelle salt.. 200 



30% soL ferrous sulphate (crystallized) 107 



(The substances must be added in the order above given and be mixed imme- 

 diately before using. It is scarcely necessary to say that distilled water must be 

 used exclusively.) As soon as the mixtures are made the iron solution is to be 

 poured into the silver aud vigorously stirred for some time. The white silver 

 tartrate becomes almost immediately bright red. then deepens in color and finally 

 becomes black. 



