Eighteenth annual Meeting. 15 



The metal was apparently perfectly passive, and maintained that appearance through- 

 out the experiment. The temperature of the solution was 20° C. Upon standing, the 

 acid began to show signs that the passivity was not complete, at the end of an hour 

 the amount of iron in solution being sufficient to color the acid unmistakably. Rep- 

 etitions of the experiment always with the same result led to the conclusion that at 

 least in 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 containing passive iron in the strong nitric acid was placed in a 

 water bath, and gradually heated. Rise of temperature was found to increase the 

 rapidity of the reaction without evolution of gas. This reaction was gradually sup- 

 planted by the second reaction described by Ordway — with elimination of hydrogen 

 — effervescence becoming very evident at 60° C. Under these conditions the metal 

 certainly could not be considered 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 acts upon "passive" iron in away similar to that 

 in which very dilute acid attacks the active metal. The transition between these two 

 reactions was not sharply marked. Further heating increased the effervescence, until 

 within the neighborhood of 80° nitrous fumes began to be evolved. The reaction 

 was still far from reaching 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 

 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 beginning 

 of a reaction of fairly explosive violence. Repetitions 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, the strength of the acid, and the character of the iron. Even the presence 

 of small quantities of the salts of iron produced by these reactions was found to in- 

 fluence in a marked degree the temperature at which in a subsequent trial the metal 

 became active. 



3. — The influence of time. 



The influence of the time of exposure upon the temperature of loss of passivity 

 was first noticed during the experiments 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. Repeated 

 trials showed that continued exposure always lowered the temperature of loss of 

 passivity. For the purpose of determining the extent of this variation, other con- 

 ditions were kept as nearly uniform as possible, and the iron was exposed for different 

 intervals of time to the acid before the destruction of passivity. The control of 

 conditions but imperfectly understood was exceedingly difficult, and the results 

 showed irregularities which would doubtless disappear were the time the only source 

 of variation. The series given in the following table is thought, however, to indicate 

 with a fair degree of accuracy the influence of the time of exposure upon the tem- 

 perature of loss of passivity. The time given in the table is that which elapsed be- 

 tween placing the iron in the acid and its change to the active condition. It will be 

 seen that the temperature of transition diminishes as the time of exposure increases. 



