256 EIGHTH REPORT — 1838. 



or iron as nearly pure as possible. Air or oxygen dissolved in 

 water is in a condensed state, and hence in a condition pecu- 

 liarly appropriate to combination. Rain water- frequently con- 

 tains, when fresh fallen, one-fifth of its volume of oxygen. 



8. When a piece of iron is immersed in such water, the whole 

 becomes electrically excited. The water, rendered more nega- 

 tive by contact with the iron, repels its dissolved oxygen, while 

 the iron, become more positive by the contact of Avater, exer- 

 cises an unusual affinity for the oxygen. Supposing the surface 

 of the metal everywhere uniform, a film of oxide is soon pro- 

 duced over it, and this once efi^ected, decomposition proceeds 

 with increased rapidity ; for as every metal is positive with re- 

 gard to its own oxides, it follows that the film of rust and the 

 iron beneath now form a voltaic couple of greater energy than 

 the last ; and whereas the electric energies were before only 

 sufficient to bring the dissolved oxj'gen of the water into com- 

 bination Avith the iron, they now become sufficient to decompose 

 the water itself, and hydrogen commences to be evolved. At this 

 epoch, if the volume of water be not too great in proportion to 

 the iron, and the latter present a large surface, as in the pre- 

 paration of ^thiop's Martial, considerable heat even is evolved, 

 but the water is previously decomposed in the cold. (Guibourt, 

 Jour, de Pharm. 1818.) It is a most remarkable JFact, how- 

 ever, that while iron has this vigorous action on water holding 

 air in solution, neither the metal nor its peroxide have any on 

 the eau oxygen^ of Thenard. — (Thenard, Triiit^.) 



9. Should the surface of the iron not be uniform in the first 

 instance, as when patches of rust pre-exist upon it, or when 

 one part is much harder or denser than another, these form 

 voltaic elements from the beginning and aid the progress of 

 oxidation. In nearly all specimens of wrought iron, when ex- 

 posed to the action of water holding air in solution, in addition 

 to the first coat of rust, one of carbon and sometimes, in mi- 

 nute quantity, of oxides higher in the electro-negative scale, are 

 deposited upon its surface, which still further exalt the condi- 

 tions favouring corrosion. 



10. When iron is freely exposed to air and water in a shallow 

 vessel, the result of their reaction is a hydrated peroxide ; if, 

 howevei*, the surfaces of the iron are placed near, but not in 

 contact with neutral solids, as glass or porcelain, or the depth 

 of water be considerable, there is also formed a large proportion 

 of magnetic oxide. Becquerel considers this difference to be 

 owing to the increased slowness of action in the latter case, 

 from the greater depth to which the water has to carry the 

 oxygen. 



