Allotroplc Silver. 499 



distilled water, the colour changes to a deep Nile-green, and 

 at the same time it becomes slightly soluble, giving a port- 

 wine coloured solution. With more washing, this solubility 

 may disappear. 



It is a general fact that all these forms of silver, however 

 various their colour, have both a body- and a surface-colour ; 

 and these two colours tend always to be complementary. The 

 body-colour is that shown by the precipitate while still moist ; 

 it is also visible when a thin coat is brushed over paper, a coat 

 so thin that light passes through it, is reflected by the paper, 

 and returned again through the film. But when a thick and 

 opaque film is applied, the body-colour disappears, and only 

 the complementary surface-colour is visible. 



So in the case of the precipitate by phosphate, when the 

 substance resembling ruby copper is spread thickly on paper, 

 it dries with a bright-green metallic surface-colour. But 

 when the substance itself becomes green by continued washing, 

 it assumes, on drying, a dark gold or copper colour, the 

 surface-colour changing with the body-colour and maintain- 

 ing its complementary relation. In observing these effects 

 one is constantly reminded of certain coal-tar colours, both 

 by the great intensity of coloration and by the metallic surface- 

 colour. I am not aware that any other inorganic substance 

 shows this resemblance to a similar extent. 



These forms of allotropic silver have a great tendency to 

 attach to themselves foreign matters. Although the dry sub- 

 stance has all the appearance of a pure metal, it may contain 

 as much as 8 or 10 per cent, of organic matter, which it is 

 utterly impossible to detach. I have taken much trouble to 

 eliminate this substance. In one attempt hot distilled water 

 was forced through with the aid of a filter-pump for over 

 one hundred hours, without effect. The presence of this 

 organic substance becomes evident when the metal is heated 

 in a tube. A vapour arises which condenses into small 

 brownish drops having an empyreumatic odour. The residue 

 of bright white metallic silver, when dissolved in nitric acid, 

 leaves behind black flakes of carbon. When the allotropic 

 silver is dissolved in dilute nitric acid and the silver preci- 

 pitated by hydrochloric acid, on evaporation a small residue 

 of a yellowish gummy substance is obtained. 



Analyses. — Four silver determinations were made of ma- 

 terial rendered as pure as it was found possible to obtain it. 

 Kesults : — 



No. 1 93'77 per cent. Ag. 



„ 2 94-27 „ 



„ a 92-86 



„ i 96-64 „ 



2N2 



