M. Carey Lea — Allotropic Forms of Silver. 479 



diluted with ten times its bulk of water, was capable of acting on 

 finely divided normal silver and of dissolving an easily recog- 

 nized quantity. Y. d. Pfordten's conclusions were thus vitiated 

 entirely. It should however be remarked that the difficulties 

 of the subject are extremely great. In his last paper* this 

 chemist abandons his views as to the existence of silver hemi- 

 oxide ; so that at the present time the formation of Ag 4 by 

 Wohler's method, or by any other known metnod, is admitted 

 by no one. That such an oxide may exist appears by no means 

 improbable. The existence of Ag 2 Cl and K 2 C1 seems almost 

 to involve that of Ag 4 and K 4 0. This latter product Davy 

 believed that he had obtained. The black substance which Y. 

 d. Pforclten formerly regarded as Ag 4 he now takes to be 

 silver hydrate Ag 4 H 2 0. 



The reduction products described by Y. d. Pfordten are 

 strongly distinguished from those which I shall presently 

 describe by two decisive reactions : — 



1. None of his products could be amalgamated with mercury 

 (1. c, 2296). All of mine readily amalgamate. 



2. None of my products give off the slightest trace of gas when 

 treated with dilute sulphuric acid. All of his do so. (1. c, 2291.) 



Moreover, the difference of appearance is extremely great. 



Early in the year 1886, I took up the study of the reduction 

 products of silver in connection with that of the photosalts. I 

 commenced with Wohler's process, giving it up after a few 

 trials as affording no satisfactory results, and sought for a more 

 reliable means. This I found, in March 1886, in a reaction 

 which I still use; namely the reduction of silver citrate by fer- 

 rous citrate. At first, however, the results obtained were most 

 enigmatical, the products very unstable, and impossible to 

 purify. Much time was lost and the matter was given up more 

 than once as impracticable. Eventually, by great modifications 

 in the proportions, stable products, and capable of a fair amount 

 of purification were got. Even the earlier and less pure forms 

 were exceedingly beautiful ; the purer are hardly surpassed by 

 any known chemical products. 



The forms of allotropic silver which I have obtained may be 

 classified as follows : — 



A. Soluble, deep red in solution, mat lilac, blue, or green whilst 

 moist, brilliant bluish green metallic when dry. 



B. Insoluble, derived from A, dark reddish brown while moist, 

 when dry somewhat resembling A. 



C. Gold silver, dark bronze whilst wet, when dry exactly 

 resembling metallic gold in burnished lumps. Of this form there 



* Ber. D. Ch. Ges., xx, 2288. 



