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LXI. Allotropic Silver. — Part III. Blue Sliver, soluble and 

 insoluble forms. By M. Carey Lea*. 



WHEN my first paper on the subject of allotropic silver 

 was published about two years ago, that product 

 seemed to be the result of a very limited number of reactions 

 closely allied to one another. Further study has shown that 

 it is a much more common product than at first appeared to 

 be the case. Wherever in the reduction of silver a reddish 

 colour shows itself, that may be taken as a probable indication 

 that allotropic silver has been formed, even although it may 

 be destroyed before it can be isolated. 



What is rather remarkable, is that allotropic silver is pro- 

 duced abundantly in certain very familiar reactions in which 

 its presence has never been suspected : so abundantly, in fact, 

 that some of these reactions constitute the best methods of 

 obtaining silver in the soluble form. In photographic opera- 

 tions silver has often been reduced by tannin in the presence 

 of alkalies. It has not been imagined that by slightly varying 

 the conditions, the whole of the silver may be made to pass 

 into solution as a soluble metal with its characteristic intense 

 blood-red colour. 



Some of these new reactions will be here described in 

 detail. 



Allotropic Silver formed by Dextrine and Alkaline 

 Hydroxide. 



When dextrine is dissolved in a solution of potassium or 

 sodium hydroxide, and silver nitrate is added, keeping the 

 hydroxide in moderate excess, the silver is at first thrown 

 down in the form of the well-known brown oxide. This brown 

 colour presently changes to a reddish-chocolate shade, and at 

 the same time the silver begins to dissolve. In a few minutes 

 the whole has dissolved to a deep-red colour, so intense as to 

 be almost black. A few drops poured into water give it a 

 splendid red colour of perfect transparency. Examination 

 with the spectroscope leaves no doubt that we have to do with 

 a true solution. 



It is interesting to observe that silver can be held in solu- 

 tion in neutral, acid, and alkaline liquids. In the first process 

 which I published, in which silver citrate is reduced by a 

 mixture of sodic citrate and ferrous sulphate, the latter may 

 be used either in acid solution, or it may be first neutralized 

 with alkaline hydroxide, so that that form of silver is held in 

 solution in either a neutral or an acid liquid. The form that 

 * Communicated by the Author. 



Phil. May. S. 5. Vol. 31. No. 193. June 1891. 2 N 



