J. C. Blake — Colors of Allotropic Silver. 283 



spreading on glass or other material, by undistarbed sedimenta- 

 tion from water, by burnishing, or by crystallization. In pro- 

 portion as the mirror surface becomes more and more imper- 

 fect, the reflected light of "white silver" changes toward 

 gray, the reflected light of "blue silver" changes to copper- 

 colored, the substance finally appearing black. The light 

 reflected by "yellow silver" has been observed only in the case 

 of mirrors on glass, backed by bone-black. Indeed, " yellow 

 silver" has been obtained satisfactorily in no other form. All 

 four forms of silver have been obtained suspended in water, 

 but suspensions of "blue silver" and "red silver" alone are 

 permanent — the so-called colloidal solutions. 



"White silver" is formed by treating "blue silver" and 

 "red silver "with strong acids in considerable amount, and, 

 consequently, whenever silver is thrown out by reduction 

 in strongly acid solution. E-oughly speaking, the greater the 

 concentration of the acid present the more nearly white the 

 silver will be. It is ordinarily gray, as noted by Lea in the 

 transformation of his allotropic forms of silver to the " ordi- 

 nary " form by treatment with acids. Silver nitrate reduced 

 by ferrous sulphate in solutions sufiiciently dilute gives a 

 colloidal solution of " blue silver." In stronger solutions a 

 gray, opaque precijDitate is formed ("white silver"). If, how- 

 ever, a little sulphuric acid be first added to the ferrous sul- 

 phate solution, the product is plainly crystalline silver consist- 

 ing of perfect and distorted microscopic octahedra ; and the 

 greater the dilution and the stronger the acid, within the 

 limits tested, the more distinctly crystalline the silver will 

 be and the more its color will approach to white. Such 

 crystals are usually intermixed with grape-like clusters which, 

 if crystalline, show no evidence of it in their external form. 



" Blue silver " may readily be obtained in a great variety of 

 ways, as indicated in Table I. It is, in fact, formed whenever 

 silver is reduced in neutral or alkaline solution in the presence 

 of small amounts of electrolytes and without the presence of 

 too much organic matter. When electrolytes in sufiicient 

 amount are added to colloidal solutions of " blue silver " aggre- 

 gation and subsequent sedimentation take place, the coagulum 

 appearing blue or black when settled in mass according to its 

 compactness and volume, the addition of salts and alkahs in 

 sufiiciently large amount tending to produce the black eft'ect. 



When such a blue or black precipitate of "blue silver" is 

 spread upon glass while moist, the particles arrange themselves 

 in mirror surfaces. If 'the preparation is made and handled 

 in the dark, these mirrors reflect a very deep and rich golden 

 color, transmitting blue light, '^o preparation was found 

 which gave these mirrors in better form than those obtained 



