78 Intelligence and Miscellaneous Articles. 



pended in the liquid so that it does not touch the platinum, the 

 attack is induced at the end of a few seconds. 



(5) At the bottom of a cylindrical vessel a small fragment of iron 

 is deposited (a Paris tack, for instance), and covered with fuming 

 nitric acid ; after a few moments of contact, passivity being induced, 

 the monohydrated acid is decanted and replaced by the dilute acid ; 

 an exactly similar fragment, but not passive, is then cautiously in- 

 troduced by means of a pincer consisting of two pieces of glass, 

 and arranged parallel to the first and at a distance from it rather 

 considerable in comparison with their diameters. The new frag- 

 ment is attacked, gas being liberated ; the other is unaltered. If 

 they be brought nearer together, at a given moment, when the dis- 

 tance is reduced to a certain amount, the passive iron enters into 

 solution. All other things beiug equal, the greater the diameters 

 of the fragments the greater is this maximum distance. 



(6) Some iron which has been rendered passive being plunged 

 into the dilute acid is rapidly attacked if a strip of metal attackable 

 by the acid is placed near it but not in contact (a metal electroposi- 

 tive in relation to iron, such as zinc). I have observed that the 

 contact of the immersed part of the iron with a copper wire was 

 sufficient to induce the reaction of the acid upon the iron ; but if 

 the contact took place upon the part which was out of the liquid, 

 no such reaction was produced. Is not the cessation of passivity 

 to be attributed to the liberation of gas produced under the former 

 conditions ? 



III. It follows from all the experiments above referred to, that 

 any shaking produced in the vicinity of the passive metal, either by 

 a shock or a vibration, or by a current of gas (sometimes very 

 slight, as in the case of spongy platinum), is sufficient to cause the 

 passivity to disappear. On the other hand, the monohydrated acid 

 exerts an action upon the metal ; but that action immediately ceases, 

 the phenomenon being interpreted by the disposition around the me- 

 tallic fragment of a gaseous enveloping sheath. We are thence led 

 to conclude from these results of experiment that this gaseous 

 sheath is the sole obstacle to further attack, that it is more adhe- 

 rent to a smooth surface and to a sample possessing great molecular 

 condensation than to one that is uneven and less compact, and that 

 mechanical shakings and gaseous currents either weak or strong 

 (the latter perhaps adding in certain cases a chemical action to their 

 displacing influence) more or less rapidly determine its dislocation. 



Experience has confirmed these previsions to which the preceding 

 trials necessarily led. If the passivity of the metal is the conse- 

 quence of the formation of the gaseous sheath, this will disappear 

 in a vacuum, and the passivity with it. A fragment of iron ren- 

 dered passive was placed in vacuo by means of peculiar arrange- 

 ments, so as to avoid any shaking. A vacuum having been produced 

 (7i = 15 millims.), the piece of iron was cautiously withdrawn with- 

 out directly touching it, and immersed in the dilute acid, where it 

 was immediately attacked. 



