Passivity of Iron in Nitric Acid. 425 



mentary effervesence and by a sudden rise of the thermome- 

 ter, which effects became more and more marked as the solution 

 became hotter. Complete and permanent activity, such, namely, 

 as was capable of maintaining itself without the continued ac- 

 tion of the magnet, did not occur until a temperature of 87'5° 

 had been reached. 



(/). A repetition of experiment (a), but with a weaker 

 field. The current-strength was - 28 of that used in experi- 

 ment (a). The magnet was active throughout the experiment. 

 The temperature of transition to the active state was 84'5°. 



(</). The acid and iron were placed in a bath at 20° and 

 placed between the poles of the magnet. The current was 

 much stronger than that employed in the experiment (a). The 

 intensity of the field, determined by the method already de- 

 scribed, was 20,000H. Loss of passivity in this field occurred 

 instantly upon closing the circuit. 



{h). The acid was diluted with one-half its bulk of water, 

 and the iron, having been introduced under the protection of a 

 strip of platinum, remained passive when the latter had been 

 withdrawn. The temperature of the acid was 23°. The iron lost 

 its passivity instantly in a field of small intensity, and regained 

 it only when removed from the neighborhood of the magnet. 

 The very slight residual magnetism of the electro-magnet was 

 found to be sufficient to destroy passivity in this weak acid, 

 the action recurring violently whenever the tube was brought 

 between the poles of the magnet, even when the circuit was 

 open. 



Upon repeating this experiment with somewhat stronger acid 

 the strength of field requisite to the destruction of passivity 

 was found to be much greater. In three parts of acid and 

 one of water the iron remained passive under the residual mag- 

 netism of the magnet, and even in the field produced by the 

 current last employed. The tangent galvanometer, when trans- 

 mission took place, indicated 0'16 of the current which had 

 been necessary, in experiment (a), to destroy passivity in con- 

 centrated acid at 51°, and the field was approximately of half 

 the strength of that under which in the strong acid (experi- 

 ment/), iron bad remained passive to 84*5°. In four parts of 

 acid and one of water at a temperature of 23°3°, the iron re- 

 mained passive until the current-strength reached *23 of that 

 used in experiment (a). 



(i). To determine in what strength of acid iron would be- 

 come active at ordinary temperatures in the earth's field, the 

 usual quantity of iron was placed in 10 co of concentrated acid, 

 and the acid rapidly diluted with water, and with such small 

 amounts of snow as were necessary to keep the mixture at 

 23°. Dilution was continued until the iron had become active. 



