488 M. Carey Lea — Allotropic Silver. 



change from blue to yellow at about 180° C. Light also pro- 

 duces this change on blue silver. The specimens obtained by 

 different processes act very differently ; some change with a 

 few hours of strong sunlight, others require many days. 



From what has been written in this and preceding papers it 

 appears that allotropic and even soluble silver may be formed 

 in a great variety of reactions. The reducing agents may be 

 either a ferrous or a stannous salt or any one of a variety of 

 organic substances of very different constitutions. From the 

 solubility and activity of this substance and the parallelism 

 which many of its reactions show to those of silver in combi- 

 nation, I have been disposed to think that silver in solution 

 might, like silver in combination, exist in the atomic form. It 

 is certain that up to the present time we have no positive 

 knowledge of the existence of any element in the atomic form 

 as a solid. We know that four or five metals are atomic in 

 their vapors and that in iodine vapor at a certain temperature 

 the molecules separate to atoms. 



But it may be questioned whether we have not seen solid 

 elements in the atomic form without recognizing them as such. 

 There are forms of iron, nickel, cobalt and lead which exhibit 

 very remarkable properties, properties that have been hitherto 

 very unsatisfactorily explained. Lead tartrate reduced by gentle 

 ignition in a nearly closed tube and allowed to cool and then 

 shaken out into the air forms a stream of fire. The oxides of iron, 

 nickel and cobalt reduced in closed tubes by hydrogen show 

 similar properties. It is customary to explain this action by 

 affirming that the metals are left in an extremely fine state of 

 division. This explanation is not satisfactory. Sulphur, for 

 example, is far more inflammable than any of the metals just 

 mentioned and may be obtained in a state of exceedingly fine 

 division, either by sublimation or by precipitation, but does 

 not in consequence show any greater tendency to spontaneous 

 inflammation. It seems more natural to suppose that these 

 metals are reduced in the atomic form, and this view of 

 the matter seems to be much strengthened by the following 

 considerations. 



The experiments of Eamsey, and of Heycock and Neville, 

 cited in a previous paper, lead to the conclusion that in the 

 case of a dilute solution of one metal in another the dissolved 

 metal exists in the atomic form. But still more the experi- 

 ments of Tammann on amalgams indicate that in these alloys 

 the dissolved metal is atomic, and it is stated that Joule by 

 distilling off the mercury from an iron amalgam found that the 

 iron was left in a pyrophoric condition. The amalgam of man- 

 ganese, carefully distilled, gives a pyrophoric powder. Chro- 



