M. Carey Lea — Allotropic Forms of Silver. 487 



When the blue substance prepared in either way dries more 

 slowly in lumps the result is very variable ; sometimes it is 

 bright bluish metallic ; sometimes dull lead color, with a 

 metallic reflection only where it has dried against a smooth 

 surface. 



B. Insoluble Form of the foregoing. 



The solution of the blue product just described is influenced 

 in a remarkable way by the addition of almost any neutral 

 substance. So far I have not found any that does not precipi- 

 tate it. Not only saline solutions do this, but even a solution 

 of gum arabic. 



Neutral salts may precipitate the silver in either a soluble 

 or an insoluble form. Alkaline sulphates, nitrates and citrates 

 throw down the soluble form, magnesium sulphate, cupric sul- 

 phate, ferrous sulphate, nickel sulphate, potassium bichromate 

 and ferro cyanide, barium nitrate, even silver nitrate and other 

 salts throw down a perfectly insoluble form. The soluble form 

 constitutes a blue or bluish black precipitate ; the insoluble, a 

 purple brown, which by repeated washing, by decantation or 

 otherwise, continually darkens. 



What is very curious is that the insoluble form may be made 

 to return to the soluble condition. Many substances are capa- 

 ble of effecting this change. Sodium borate does so, producing 

 a brown solution, potassium and sodium sulphate produce a 

 yellowish red solution and ammonium sulphate a red one. 

 None of these solutions has the same blood-red color as the 

 original solution ; the form of silver seems to change with the 

 slightest change of condition. 



The solutions used must" be extremely dilute, otherwise the 

 silver, though rendered soluble in pure water by them, will 

 not dissolve in the solution itself, a singular complication of 

 effects. So that if a moderately strong solution of one of the 

 above substances is poured over the insoluble silver substance 

 it does not dissolve, but by pouring off the saline solution and 

 replacing it with pure water, the substance now dissolves 

 readily. The insoluble substance is also readily soluble in 

 ammonia. The solution has a tine red color, and not the yel- 

 lowish red of the sodium sulphate solution. 



Most neutral salts act in one or other of the ways just 

 described, precipitating the solution of the blue substance A 

 in either the soluble or the insoluble form, the latter soluble in 

 ammonia, but sodium nitrite is an exception ; its solution 

 effects an entire change and renders the substance wholly 

 insoluble, probably reconverting it to normal silver. 



Km. Jour. Sci.— Third Series, Vol. XXXVII, No. 222.— June, 1889. 

 31 



