Intelligence and Miscellaneous Articles. 11 



render the iron passive. The experiments which I have the honour 

 to submit to the Academy were instituted with the aim of casting 

 some light on this interesting question. 



I. (1) Iron which has been rendered passive is immersed in the 

 ordinary acid and caused to strike against the sides of the vessel 

 containing it, the necessary energy of the shock being the less the 

 more dilute the acid ; solution takes place instantaneously. The 

 same result can be attained either by rubbing the metal with a glass 

 rod, or by projecting a jet of water upon it while immersed, or by 

 setting it in rapid gyration. 



(2) Vibrations even of extremely slight amplitude suffice to make 

 the passivity cease. M. Ducretet has been so good as to construct, 

 according to my indications, a very simple apparatus for the study 

 of this novel influence. A strip of soft iron is interrupted in its 

 middle by a strip of caoutchouc. One of the parts is fixed verti- 

 cally in apincer; the lower part is completely immersed in the 

 fuming acid and thus rendered passive. The monohydrated is re- 

 placed by commercial acid, in which the iron remains intact. Vibra- 

 tions are then induced by bringing near to the upper part an elec- 

 tromagnet similar to those employed for electric bell signals. 

 Scarcely has the vibratory motion commenced before the reaction 

 appears. 



II. On carefully examining with a powerful lens a fragment of 

 rugous metal immersed in the fuming acid, I saw a gaseous sheath 

 form about the iron, which persists when it is put into the dilute 

 acid. If the fragment is very smooth and compact, this sheath is 

 more difficult to perceive, but its formation is nevertheless apparent. 

 I was led by this observation to study the part which might be 

 played by the gaseous envelope in the phenomenon with which we 

 are occupied. The following experiments were made for that pur- 

 pose. 



(1) Iron rendered passive is entirely immersed in the dilute acid, 

 then cautiously taken out so as to be completely covered with the 

 acid liquid, and suspended in the air. After a few moments (almost 

 immediately in a rather brisk current of air) the attack commences, 

 and continues with energy. It is preceded by the sudden appear- 

 ance of gas-bubbles coming to the surface and bursting. 



(2) Iron rendered passive is plunged into dilute nitric acid; 

 some air-bubbles are caused to arrive in the vicinity of the metallic 

 fragment ; the reaction commences, being set up the more quickly 

 the more uneven the surface of the metal. 



(3) Under the same conditions, the metal being suspended in the 

 liquid, some pieces of a carbonate (chalk, for example) are deposited 

 at the bottom of the vessel ; carbonic acid being liberated, the metal 

 is attacked. 



(4) A vessel containing dilute nitric acid is prepared beforehand, 

 on the bottom of which a little spongy platinum or platinum-black 

 is placed (substances eminently porous and liberatiug on immersion 

 numerous gas-bubbles); a piece of iron rendered passive being sus- 



