Passivity of Iron in Nitric Acid. 421 



the case of finely divided " iron by alcohol," in strong acid, 

 Ordway's third reaction (the solution of the iron without the 

 evolution of gas) was always going on. 



(2). On the destruction of passivity by heat. — A test tube con- 

 taining passive iron in strong nitric acid was placed in a water- 

 bath and heated. Rise of temperature was found to increase 

 the rapidity of the reaction without evolution of gas. This 

 was gradually supplanted by the second reaction described by 

 Ordway (with the elimination of hydrogen), effervesence becom- 

 ing very evident at 60°. Under thase conditions the metal 

 could not be regarded as " passive" in the strict sense of the 

 word, and yet the reaction differed entirely from that of strong 

 acid upon active iron. At this temperature the concentrated 

 acid acted upon the passive iron in a way similar to that in 

 which a very dilute acid attacks the active metal. 



The transition between these two reactions was not sharply 

 marked. Further heating increased the effervesence until, with- 

 in the neighborhood of 80°, nitrous fumes began to be evolved. 

 The reaction was still far from equaling that which such acid 

 is capable of exerting upon active iron, but the condition of 

 the solution became more and more unstable, and a slight 

 further rise of temperature sufficed to render the iron subject 

 to the full effect of the acid. This final and complete loss of 

 passivity was very sharply marked by the setting in of a reac- 

 tion of fairly explosive violence. 



Eepetitions of this experiment showed that the temperature, 

 at which these successive changes occur, depends upon a variety 

 of circumstances. It was found to vary with the time the iron 

 had been exposed to the acid, with the strength of the latter, and 

 with the character of the iron. Even the presence of small 

 quantities of the salts produced by these reactions was found 

 to influence in a marked degree the temperature at which, in a 

 subsequent trial, the metal became active. 



(3). On the influence of time. — The influence of the time of 

 exposure upon the temperature of the loss of transition was 

 first noticed during the experiment just described. A test-tube 

 containing passive iron in nitric acid had been allowed to stand 

 for some time before being placed in the water-bath. Instead 

 of retaining its passivity to a temperature of nearly 90°, this 

 specimen became active at a much lower temperature. Re- 

 peated trials showed that continued exposure always lowered 

 the temperature at which passivitj^ was lost. To determine 

 the extent of this variation, other conditions were kept as 

 nearly uniform as possible and the iron was exposed, for differ- 

 ent intervals of time, to the acid before the destruction of its 

 passivity. The control of conditions but imperfectly understood 

 was far from complete, and the results of the determination 



