THE SMITHSONIAN INSTITUTION. 419 



calternately destroyed and reproduced, so that at/:) a pulsating develop- 

 ment of gas takes place. 



The length and thickness of the wires which produce the effect 

 descrihed, depend upon circumstances. SchOnhein in his experiments 

 obtained with a copper wire 3 inches long, and ^ an inch thick, a 

 constant liberation of gas at p. A wire 40 feet long of the same 

 thickness did not destroy the passivity of e. A wire of the same 

 thickness and 16 to 20 feet long produced the pulsations mentioned 

 above. After closing, a short time elapsed before the gas began to 

 appear at j9 ; it was more lively than that which was produced by 

 shorter wires, but ceased again after a few seconds, and soon began 

 again. This alternate action and inaction continued, until at last a 

 constant state of inaction occurred. (Fog. Ann. LVII, 63.) 



§52. Theory of passivity. — Upon a review of the foregoing facts, 

 the theory of passivity can hardly be doubtful ; it will appear readily 

 from the general phenomena, though there are many single facts which 

 need closer investigation. 



It may be considered certain, that the phenomena of the passivity 

 of iron are induced by a film of oxide or sub-oxide which on the one 

 hand protects the iron from the attack of the acid, and on the other 

 acts as an electro-motor, like the film of hyper-oxide of lead, which 

 covers a platinum plate. 



The constitution of this film, and the conditions under which it is 

 formed and dissolved, are indeed questions which cannot in all cases be 

 satisfactorily answered, yet that is not a sufficient reason for rejecting 

 the basis of explanation alluded to above. 



The formation of the oxide film in heating iron red-hot is clear. 

 To form a similar film by immersion in a liquid, it is necessary that 

 the requisite quantity of oxygen should be conveyed to the iron before 

 any other chemical action of the liquid on the iron can take place. 



Concentrated nitric acid is so rich in oxygen, that mere immersion 

 of iron in it suffices to form the film. How ithappens^, however, that 

 an iron wire becomes passive by repeated immersion in acid of the 

 sp. gr. 1.35 is not yet clearly explained. 



In liquids which contain less oxygen a galvanic current must sus- 

 tain the communication of oxygen to the iron, in order to form the 

 film, and thus, the electro-motive force generating the current must 

 be the stronger the less easily oxygen can be liberated from the liquid. 

 In nitric acid of sp. gr. 1.35, the combination of the iron wire with 

 platinum suffices ; but with dilute sulphuric acid a voltaic pile must 

 be used. 



That an iron wire which has been rendered passive 'bj mere im- 

 mersion in concentrated nitric acid, or by combination with platinum 

 in dilute nitric acid, should exhibit its perfect metallic lustre, is no 

 just reason for doubting the presence of a thin film of oxide in this 

 case, for such films must, at increasing thicknesses, pass through the 

 difterent shades of Newton's ring's ; then, so Ions; as the film has 

 only a thickness corresponding to the colors of the first order, it can 

 impart to the metallic lustre of tlue wire, at most, only a feeble 

 shading into blue or yellow. 



