THE SMITHSONIAN INSTITUTION. 421 



distilled water, and the surface rubbed one or two minutes with clean 

 blotting paper, the plate, after drying, will act negatively towards a 

 second with which it was at first homogeneous. 



3. If an iron plate bo heated over a spirit-lamp to an imperceptible or 

 invisible iridescence, it will act, after cooling, very strongly negative 

 towards a plate not thus treated, so that three contacts will suffice for 

 completely charging the condenser. Such a plate acts negatively 

 towards copper, silver and gold. 



§ 53. Passivity of other metals. — Other metals, especially bis- 

 muth, copper, and tin, manifest similar phenomena of passivity, 

 though in a less marked degree than iron. Andrews (Pog. Ann. 

 XLV, 121) made the observation that a small piece of bismuth which, 

 was immersed in a large quantity of nitric acid of the sp. gr. 1.4, and 

 then brought into contact with a platinum plate in the liquid, almost 

 wholly ceased to dissolve, and at the same time took on a peculiar 

 lustre, while the same metal alone would be attacked violently by the 

 acid. 



When a small rod of bismuth was made the positive pole of a small 

 battery of two pairs of Grove's elements, and immersed in nitric acid 

 of the sp. gr. 1.4, its solubility was at once diminished, and upon 

 breaking its connexion with the battery, it showed itself to be in the 

 passive state. 



The solubility of bismuth is not totally destroyed in its passive 

 state, as is the case with passive iron ; it is only altered in degree. 

 When it forms the positive pole of a battery, bismuth does not develop 

 free oxygen, (Schonbein in Pog. Ann. XLIII,) as is the case with, 

 passive iron ; but it is dissolved, though slowly, if a weak battery is 

 used ; more rapidly with a strong one. 



Therefore, the protecting envelope of oxide acts similarly on bis- 

 muth as on iron, though its protecting power is less on the former. 



Andrews observed the same kind of phenomena in tin and copper. 



Beetz remarks (Pog. Ann. LXVII, 210,) that the reason why iron 

 is particularly disposed to passivity is probably to be found in the 

 great electrical difference between iron and its oxide. According to 

 this view, a metal should exhibit the phenomena of passivity more de- 

 cidedly as the electro-motive force between it and its oxide is greater. 



NOTES. 



(See page 323.) It might at first sight be supposed that the deflect- 

 ing forces in the two positions Fig. 4 and Fig. 5, ought to be equal ; 

 but that the deflecting force in the latter position should be double of that 

 in the former, is explained by the fact that in the position Fig. 4, the de- 

 flecting force is determined by the difference of direction of the attract- 

 ing and repelling poles of the deflecting magnet from each pole of the 

 deflected magnet, without difference of distance ; while in the position 

 Fig. 5, it is determined by the difference of distance of the same at- 

 tracting and repelling poles from each pole of the deflected magnet, 

 and the attraction and repulsion are inversely not as the first power, 



