M. Carey Lea — Allotropic Forms of Silver. 489 



destroyed the crystallization and the substance dried with a 

 bright green metallic luster. Contact with pure water evi- 

 dently tends always to bring this form of silver into the col- 

 loidal state, sometimes soluble and sometimes not ; whilst the 

 contact with certain neutral salts renders it crystalline. 



The extraordinary sensitiveness which allotropic silver shows 

 to external influences coutrasts strongly with the inertness of 

 normal (probably polymeric) metallic silver. When we place 

 this fact alongside of the well known sensitiveness of many 

 silver compounds to light, heat and (as I have elsewhere 

 shown) to mechanical force,* we are led to ask whether silver 

 may not exist in this form in these very sensitive compounds. 



To obtain the substance in a pure condition suitable for 

 analysis, it is necessary to choose a precipitant not giving an 

 insoluble product with either citric or sulphuric acid. Magne- 

 sium sulphate or nickel sulphate answers well ; I have generally 

 used the first named. A very dilute solution is made of it and 

 the red solution of A is to be filtered into it. The precipitate 

 soon subsides. A large quantity of water is to be poured on, 

 and then washing by decantation can be continued to three 

 decantations, after which the substance remains suspended. It 

 can be made to subside by adding a very small quantity of 

 magnesium sulphate ; one four-thousandth part (0*25 gram to 

 one liter) is sufficient. The substance may then be thrown on 

 a filter and washed with pure water. 



Analysis. — A specimen dried in vacuo over sulphuric acid 

 gave 



No. 1 97"17 per cent silver. 



No. 2 97-10 " " 



A specimen dried first in vacuo and then at 100° C, lost in 

 the second drying '88 per cent water. 



So that the substance dried at 100° contained 97*96 per cent, 

 of silver. The remaining 2*04 per cent consisted of ferric 

 oxide and citric acid. 



G. Gold- Yellow and Gopper-colored Silver. 



It has been long known that golden-yellow specks would 

 occasionally show themselves in silver solutions, but could not 

 be obtained at will and the quantity thus appearing was infi- 

 nitesimal. Probably this phenomenon has often led to a sup- 

 position that silver might be transmuted into gold.f This yel- 



* Production of an image on silver iodide capable of development by simple 

 pressure. 



f I have a little volume published in Paris in 1851 by a chemist named Tiffe- 

 reau 'who was firmly convinced that in many reactions, minute portions of silver 

 are converted into gold, especially with the aid of powerful sunlight. In Mexico, 

 he affirmed, he had artificially produced several grams of gold, a portion of 



