284 J. C. Blake— Colors of Allotropic Silver, 



by following Lea's directions for the preparation of his " gold 

 silver" ; that is by the action of Rochelle salt and ferrous sul- 

 phate on a solution of silver nitrate. Drained and dried 

 quietW in masses, these preparations give Lea's golden lumps. 



Heat converts this blue silver to " white silver." Pressure 

 does the same, and with great ease in the case of mirrors on 

 glass. Exposure to liglit gradually brings about the same 

 effect, the golden reflection slowly paling. If such mirrors of 

 " blue silver " are originally prepared in the daylight, the sur- 

 face reflection at first is pale yellow, wdiich likewise fades into 

 white by lapse of time. This partially converted yellow reflect- 

 ing silver is Lea's " intermediate " form, scarcely sensitive to 

 pressure. Its pale yellow color may reasonably be attributed 

 to the dilution of the golden yellow surface-reflection of "blue 

 silver " by the white light reflected by intermingled " white 

 silver " formed by the exposure to daylight. 



Mirrors of " red silver " and of " yellow silver " on glass 

 may be conveniently prepared by the action of silver nitrate 

 upon an ammoniacal sohition of tannic acid, and mirrors of " red 

 silver" may likewise be readily obtained by spreading on glass 

 portions of the red-brown precipitate formed in the mixture. 

 The mirrors of '• yellow silver " thus formed tend to change 

 into "red silver" spontaneously, and both, when gently heated, 

 are transformed into "blue silver." These mirrors of both 

 forms of silver are slowly soluble in water, but mirrors of 

 " red silver " are stable for some weeks under atmospheric 

 conditions, ultimately becoming dull gray and lusterless. 



Colloidal solutions of " red silver " of approximate purity 

 can be readily obtained by reducing a solution of silver nitrate 

 with ferrous citrate (ferrous sulphate and sodium citrate) in the 

 presence of a little free alkali, according to Lea's method of 

 obtaining his " A " form of silver. The mother liquor should 

 be withdrawn by suction through a porous cell, as recommended 

 by Schneider, and the red precipitate suspended in water. If 

 no free alkali is added the solution will be blue (experiment 

 (19) of the table), a trace of free alkali in excess rendering 

 most collodial solutions more stable. On adding an electrolyte 

 in sufficient amount to such a red solution, the silver is changed 

 to " blue silver " and usually settles out. If the yellowish 

 brown mother liquor has not been removed, the addition of an 

 electrolyte will cause the solution to look green instead of blue, 

 owing to the commingling of colors. To the intermixture of 

 yellow sulphur is due the blue-green effect observed in experi- 

 ments (39) and (48) of the table. " Red silver " may be looked 

 for whenever silver is thrown out by reduction in the absence 

 of electrolytes, or in the presence of electrolytes in small 

 amount, accompanied by considerable amounts of organic 



