500 Mr. M. Carey Lea on 



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 

 carbonated 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 colour of this solu- 

 tion is likewise very intense : one containing 1 per cent, of 

 silver is quite black, on dilution deep yellowish red. It has 

 very much the same characters as the preceding, but is rather 

 more stable. To obtain it, 24 grammes of dry sodium car- 

 bonate may be dissolved in 1200 cub. centim. of water. A 

 4-per-cent. solution of tannin is to be made and filtered ; 

 of this, 72 cub. centim. are to be added to the solution just 

 named : of silver nitrate, 24 grammes dissolved in a little water 

 are to be added by degrees. Solution takes place almost 

 instantly as each successive portion is added. The solution, 

 after standing a day or two, may be decanted or filtered from 

 a small quantity of black precipitate. 



When the solution is treated w r ith a very dilute acid, as, for 

 example, nitric acid diluted with twenty times its bulk of 

 water, allotropic silver is precipitated in the solid form. It 

 dries with a brilliant metallic surface-colour of a shade dif- 

 ferent from the foregoing, and somewhat difficult to exactly 

 characterize, a sort of bluish steel-grey. 



I do not find that the blue allotropic silver (in which is 

 included the green and steel-grey varieties) can be reduced 

 to any one definite type. On the contrary, its variations are 

 endless. Slight differences in the conditions under which the 

 solutions are formed, or in the mode of precipitation, give 

 quite different products. For example : of ten products ob- 

 tained with tannin and sodium carbonate in different propor- 

 tions, several were easily and completely soluble in ammonia, 

 some were slightly soluble, and some not at all. Some spe- 

 cimens not at all soluble in water became so by moistening 

 with dilute phosphoric acid : they did not dissolve in the acid, 

 but when it was removed they had become soluble in water. 

 On other specimens phosphoric acid had no such effect. Some 

 solutions are scarcely affected by acetic, acid, others are partly 

 precipitated, others almost, but not quite, wholly. The films 



