M. Carey Lea — Allotrojpic Forms of Silver. 481 



A. Soluble Allotropic Silver. 



A solution of ferrous citrate added to one of a silver salt 

 produces instantly a deep red liquid. (Ferrous tartrate gives 

 the same reaction but is less advantageous.) These red solu- 

 tions may either exhibit tolerable permanency or may decolor- 

 ize, letting fall a black precipitate. It is not necessary to 

 prepare the ferrous salt in an isolated form, a mixture of fer- 

 rous sulphate and sodic citrate answers perfectly. 



When, however, concentrated solutions are used with a large 

 excess of ferrous sulphate and a still larger one of alkaline 

 citrate, the liquid turns almost completely black. It should be 

 stirred very thoroughly for several minutes, to make sure that 

 the whole of the precipitated silver citrate is acted upon by 

 the iron. After standing for ten or fifteen minutes, the liquid 

 may be decanted and will leave a large quantity of a heavy pre- 

 cipitate of a fine lilac-blue color. It is best to adhere closely 

 to certain proj^ortions. Of a ten per cent solution of silver 

 nitrate, 200 c. c. may be placed in a precipitating jar. In 

 another vessel are mixed 200 c. c. of a thirty per cent solution 

 of pure ferrous sulphate and 280 c. c. of a forty per cent solu- 

 tion of sodic citrate. (The same quantity of ferrous sulphate 

 or of sodic citrate in a larger quantity of water will occasion 

 much loss of the silver product.) I think some advantage is 

 gained by neutralizing the ferrous solution, which has a strong 

 acid reaction, with solution of sodium hydroxide : as much 

 may be added as will not cause a permanent precipitate. To 

 the quantities already given, about 50 c. c. of 10 per cent soda 

 solution. The reaction takes place equally well without the 

 soda, but I think the product is a little more stable with it. — 

 The mixed solution is to be added at once to the silver solution. 



The beautiful lilac shade of the precipitate is rather ephem- 

 eral. It remains for some time if the precipitate is left under 

 the mother water, but when thrown upon a filter, it is scarcely 

 uncovered before the lilac shade disappears and the precipi- 

 tate takes a deep blue color, without losing its solubility. It 

 may be washed either on a filter or by decantation, with any 

 saline solution in which it is insoluble and which does not affect 

 it too much. On the whole, amnionic nitrate does best, but 

 sodic nitrate, citrate, or sulphate may be used, or the correspond- 

 ing ammonia salts. Although in pure water the precipitate 

 instantly dissolves with an intense blood red color, the presence 

 of five or ten per cent of any of these salts renders it perfectly 

 insoluble. I have usually proceeded by adding to the precipi- 

 tate (after decanting the mother water as completely as may be 

 and removing as much more as possible with a pipette), a 

 moderate amount of water ; for the above quantities about 150 

 c. c. Much less would dissolve the precipitate but for the 



