M. Carey Lea — Allotropie Silver. 483 



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 color. 



Some of these new reactions will be here described in detail. 



Allotropie Silver obtained with 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 color 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 color, so intense 

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

 a splendid red color 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 solution 

 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 is obtained 

 with the aid of dextrine dissolves most freely in the strongly 

 alkaline liquid in which it is produced, and when dilute nitric 

 or sulphuric acid is added the silver is precipitated. But with 

 acetic acid the precipitation is very incomplete : the solution 

 retains a brown color and contains silver. Even the addition 

 of a large excess of strong acetic acid fails to throw down any 

 more silver. It follows therefore that while this form of 

 silver is most freely soluble in a strongly alkaline liquid it is 

 also soluble to some extent in one that is either neutral or acid. 



The precipitate when once formed appears to be almost 

 insoluble. A small portion of it stirred up with distilled 

 water gives no indication of solution. But if a quantity is 

 thrown on a filter and washed, as soon as the mother water is 

 washed out the liquid runs though of a muddy red, and if this 

 filtrate be allowed to stand it deposits an insoluble portion and 

 then has a fine rose-red color and perfect transparency. Not- 

 withstanding the beautiful color it contains a trace of silver 

 only, so great is the coloring power of the metal. Sometimes 

 if the alkaline solution stands for a month or two the silver 

 becomes spontaneously insoluble ; most of it falls to the bot- 

 tom as a deep red substance, but part remains in suspension 



