Dilute Acids on some Amalgam Surfaces. 215 



by Dr. Gladstone and Mr. Tribe by the deposition of a nega- 

 tive upon a positive metal, described by them at the British- 

 Association Meeting at Bristol in 1875. 



Connected with the same subject, but susceptible probably 

 of a different explanation, is the following experiment : — 



On a clean dry surface of rich lead amalgam a drop of strong 

 nitric acid is placed. The acid spreads itself out, the amalgam 

 surface underneath it, in the course of a second or two, assu- 

 ming a dull leaden-grey colour. Suddenly, with a flash, this 

 colour changes to a whitish tint, which gradually deepens 

 again in colour until, after another second or two, another 

 flash restores the whitish tint. This alternation of colour is 

 sometimes repeated steadily many times in succession during 

 several minutes, until an energetic action at length sets in and 

 gas is evolved. 



The change from grey to white is always abrupt ; the change 

 from white to grey always gradual. 



Instead of employing a vessel with lead amalgam, the flash- 

 ing was equally well observed with the amalgamated surface 

 of a piece of common sheet-lead. A galvanometer placed in 

 the circuit showed that at the instant of the flash the covered 

 surface became less electropositive. 



A quantity of lead amalgam placed in a shallow dish and 

 completely covered with strong nitric acid showed changes of 

 colour as before. At each flash it was observed that the area 

 of the amalgam was suddenly contracted ; and it reexpanded 

 during the change of colour from white to grey. 



A drop of strong nitric acid placed upon a bright surface of 

 lead attacks it gradually, and produces a dull grey colour 

 which is slightly opalescent. 



A drop of strong nitric acid placed upon a bright surface of 

 mercury attacks it energetically, producing white crystals and 

 evolving gas plentifully. 



The behaviour of the separate materials suggests an expla- 

 nation of what may happen in the case of the amalgam, viz. 

 that while tbe surface is turning to a grey colour, the nitric 

 acid is attacking lead ; and when it suddenly flashes white, the 

 acid is attacking mercury. The strange part of the pheno- 

 menon is the apparent passivity of the mercury for some time 

 after placing the acid upon the amalgam. The explanation 

 which appears to me to be the most probable is, that the first 

 contact of the acid witb the amalgam surface results in the 

 production of a thin stratum of lead-salt, which floods the mer- 

 cury surface and tends to keep the nitric acid for a time sepa- 

 rated from it. Tbe acid in course of a second or so, however, 

 diffusing through this stratum, commences an attack upon the 



