422 Nichols and Franklin — Destruction of the 



showed irregularities which would doubtless disappear — had the 

 time been the only remaining 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 ex- 

 posure upon the temperature of loss of passivity. The time 

 given is that which elapsed between the placing of the iron-in 

 the acid and its change to the active condition. It will be 

 seen that the temperature of transition falls as the time of ex- 

 posure increases. 



Table I. 



Influence of the duration of exposure to the acid upon the temperature 

 at which iron becomes active in NHOs, sp>. gr. 1-368. 



Duration of Exposure. Temperature of Transition. 



m 45 3 89-4° 



l m 2 s 84'2° 



4 m 25 s 79-3° 



6 m 25 s 78-7° 



9 m 30 s 78-3° 



13 m 50 s 71-5° 



25 m 30 s 70-8° 



(4). On the influence of the presence of iron salts in the solu- 

 tion. — The presence of even a small amount of the final pro- 

 ducts of the action of nitric acid upon iron was found to lower 

 the temperature of transition to the active state to a marked ex- 

 tent. In order to obtain consistent results it was necessary to 

 clean the test-tubes in which the reaction was to take place, and 

 the thermometer, by boiling in sulphuric acid. Failure to 

 thoroughly cleanse the latter repeatedly reduced the temper- 

 ature of transition many degrees, in one instance to 60°, and 

 in a test-tube from which the products of a former reaction 

 had not been entirely removed, it was often impossible to ren- 

 der iron passive even at 20°. 



(5). On the influence of the strength of acid. — A more obvi- 

 ous cause of variation in the temperature of transition than 

 those already described, lies in the different strengths of acid 

 in which the iron is submerged. A set of experiments upon 

 this variation gave the following results : 



Table II. 



Temperature of transition from the passive to the active state in acid 

 of different degrees of concentration. 



Sp. Gr. of Acid. Temperature of Transition. 

 1-368 89-5° 



1-354 82-3° 



1-306 69-6° 



1-13U 23-0° 



