Allotropic Silver. 503 



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 vapours, and that in iodine vapour at a certain 

 temperature the molecules separate into 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. Sul- 

 phur, 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 preci- 

 pitation ; but does not, in consequence, show any greater ten- 

 dency 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 

 manganese, carefully distilled, gives a pyrophoric powder. 

 Chromium amalgam, distilled in a current of hydrogen, gives 

 a similar result if the temperature is not raised too high. The 

 enormous affinity which these forms of metals exhibit for 

 oxygen renders their study very difficult. It has not been 

 before suggested that their activity is due to their being 

 atomic ; but this would seem to be a much more rational ex- 

 planation than that of extreme division*. A broad distinction 

 must of course be drawn between chemical and mechanical 

 division : a substance may be atomic and yet appear in masses ; 

 may be in the finest mechanical division and yet be molecular 



* M. G. Rousseau, in the new Encyclopedie Chimique, seems to entirely 

 abandon the old view of extreme division, and considers these forms to be 

 allotropic, and comparable with the allotropic forms of phosphorus, &c, 

 Vol. iii. p. 56. 



