Dilute Acids on some Amalgam Surfaces. 213 



injection of oxygen ; and it was observed that the motions im- 

 mediately became more rapid and, at the same time, more 

 regular all round. When the supply of oxygen was stopped 

 and atmospheric air substituted, the motions resumed their 

 original irregularity. 



Exp. 4. The foregoing experiment was repeated by allow- 

 ing a drop of acid water on the amalgam surface to fall into 

 motion underneath the receiver, and then changing the atmo- 

 sphere to one of hydrogen. The motions immediately ceased. 

 On letting in the air again, the motions were resumed. In 

 carbonic acid, nitrogen, and coal-gas the motions were in- 

 stantly arrested ; and by covering the amalgam surface under 

 the receiver alternately with one of these gases and with either 

 atmospheric air or oxygen, the motions could be stopped and 

 reestablished at pleasure. 



Exp. 5. On a surface of lead amalgam a drop of dilute nitric 

 acid was placed, and the tip of a lead wire suspended in the 

 drop. Between the lead wire and the amalgam was inserted 

 a delicate reflecting galvanometer. By means of an adjustable 

 shunt across the galvanometer, the first current was reduced 

 until the index fell nearly to zero. Then the shunt resistance 

 was increased so as to obtain the greatest sensitiveness, whilst 

 retaining the light point upon the scale. In this position, 

 whilst Mr. McEniry observed the motions of the acid-drop, I 

 observed the motions of the galvanometer, each giving a signal. 

 In this way it was found that each spreading of the drop was 

 accompanied by a slight excursion of the light-point. The 

 direction of the permanent deflection showed that the amalgam 

 surface was positive to the lead wire. The direction of the 

 excursions indicated a slight diminution of this permanent 

 current, or that at the moment of expansion of the acid- 

 drop, the amalgam surface became slightly less electropositive. 

 In the intervals between the expansions, the tendency was for 

 the needle to move in the other direction. Lead is positive to 

 mercury in dilute nitric acid ; therefore this reduced electro- 

 positiveness may indicate an increased proportion of mercury 

 on the covered surface at the instant of spreading. 



The conclusion which I draw from these experiments is, 

 that the motions in question are due to a portion of the surface 

 of the amalgam becoming alternately oxidized by the air out- 

 side the acid-drop, and deoxidized by electrolysis in the inte- 

 rior of the drop. The reciprocal play of these two actions is, 

 I venture to suggest, as follows : — When the acid-drop is placed 

 upon the surface of the amalgam of a metal specifically lighter 

 than mercury, it finds the surface to consist of mercury in 

 which are floating innumerable particles of the foreign metal. 



