M. Carey Lea — Allotropic Silver. 485 



not aware that any other inorganic substance shows this resem- 

 blance 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 100 hours with- 

 out effect. The presence of this organic substance becomes 

 evident when the metal is heated in a tube. A vapor arises 

 which condenses into small brownish drops having an empy- 

 reumatic odor. 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 precipitated by hydrochloric acid, on evapo- 

 ration a small residue of a yellowish gummy substance is ob- 

 tained. 



Analyses. — Four silver determinations were made of mate- 

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

 Results — 



No. 1 93-77 p. c. Ag. 



No. 2 94-27 " " 



No. 3 92-86 " " 



No. 4 __ 96-64 " 



Allotropic Silver obtained with Tannin and alkaline Carbonates. 



Tannin (gallotannic acid) in alkaline solution reduces silver 

 nitrate to metallic silver in the allotropic form. Tannin acts 

 more strongly than dextrine and therefore does best with car- 

 bonated alkali, dextrine best with alkaline hydroxide, although 

 either substance will produce the reaction with either form 

 of alkali and, though less advantageously, with ammonia. 

 Tannin with sodium carbonate gives a very perfect solution of 

 silver, quite free from the turbidity that is apt to characterize 

 the dextrine solution. The color of this solution is likewise 

 very intense : one containing one per cent of silver is quite 

 black, by dilution deep yellowish red. It has very much the 

 same characters as the preceding, but is rather more stable. 

 To obtain it, 24 grams of dry sodium carbonate may be dis- 

 solved in 1200 cc. of water. A 4 per cent solution of tannin 

 is to be made and filtered, of this 72 cc. are to be added to the 

 solution just named : of silver nitrate, 24 grams dissolved in a 

 little water are to be added by degrees. Solution takes place 

 almost instantly as each successive portion is added. The solu- 

 tion after standing a day or two may be decanted or filtered 

 from a small quantity of black precipitate. 



