THE SMITHSONIAN INSTITUTION. 411 



1 incli wide and 1.5 deep, immersed in dilute acid, yielded 8 cuLic 

 centimetres in five minutes. 



The current of polarization which such a secondary battery yields 

 by means of the pole-changer is considerably stronger than that of a 

 Grove's gas-battery. The intermitting current of a secondary battery 

 of two pairs of plates gave in one minute with the pole-changer V* = 

 2.8 c. c. of gas, while the continuous current of a gas-battery of ten 

 cells yield only 2.1 cubic inches in thirty-six hours ; thus only about 

 0.016 cubic centimetre of gas per minute. 



§ 47. Old observations on the relation of iron to nitric acid. — On 

 immersing an iron wire in nitric acid of the specific gravity 1.4, it 

 instantly turns brown, while red vapor escapes with more or lesa 

 effervescence. This, however, soon ceases ; the iron recovers its me- 

 tallic lustre, and retains it as long as it remains in the acid without 

 being further attacked. Once placed in this state of chemical in- 

 activity, such a wire will remain so even in dilute acid, which of 

 itself could not have produced this condition. 



This remarkable relation of iron to nitric acid was observed as early 

 as the last century by James Keir, and published in the Phil. Trans, 

 for 1790 ; but the phenomenon was too much isolated to allow a true 

 determination of its nature, and thus Keir's observation was forgotten. 



After the lapse of thirty-seven years, Wetzlar made similar obser- 

 vations, which he published in Schweigger's Jahrbuch der Chemie 

 und Physik ; Bd. 49, S. 470 ; Bd. 50, S. 88 and 129 ; Bd. 56, S. 206. 

 In England, Herschell took up this subject, (Pogg. Ann., XXXII, 

 211 ; Ann. de Chemie et de Phys., 1833, vol. LIV, 87,) and Fechner 

 observed similar phenomena in the action of nitrate of silver on iron. 

 Schonbein has prosecuted this subject most zealously, and to him be- 

 longs the merit of having extended, more than any one else, the circle 

 of the phenomena relating to it. 



Since Schonbein has investigated the phenomena of the passivity of 

 iron (a term which was introduced by himself) the most thoroughly, 

 it may be advisable to take our facts chiefly from his memoirs. This 

 distinguished natural philosopher, however, will, I hope, not take 

 offence if I should venture the remark, that the peculiar diffuseness 

 which characterizes these papers renders them difficult to understand. 



§ 48. Schonbein' s observations on the passivity of iron. — His first 

 paper on this subject may be found in Poggendorff's Annalen, 

 XXXYII, 390. 



"• It has long been known," Schonbein begins, " that very concen- 

 trated nitric acid does not attack many metals, which are oxidized with 

 violence by the same acid containing more water. Of these metals 

 tin is one, but iron more especially has this characteristic. 



" An iron shaving perfectly free from rust was not attacked by 

 nitric acid of the specific gravity of 1.5. Even after adding to the 

 acid as much water as will dilute it to the degree at which it would 

 aittack fresh iron shavings violently, the shaving thus treated will re- 

 main perfectly passive. 



" It is not only the treatment with concentrated nitric acid which, 

 produces this passivity. Iron filings, heated for only a few seconds 



