Passivity of Iron in Nitric Acid. 



423 



The time of exposure to the acid was in each case about 40 

 seconds. This determination agrees well with that noted by 

 Schoenbein for iron wire in nitric acid, sp. gr. 1'35. He found 

 the change to occur at 75°. 



III. 



In these preliminary experiments the method pursued had 

 been very simple. The powdered iron was placed in a per- 

 fectly clean and dry test-tube, and the acid having been meas- 

 ured out was poured upon it. The test-tube was then placed 

 in a water-bath, a thermometer was inserted in the acid, and the 

 transition-temperature, always indicated by an explosion of 

 such violence as to expel the greater portion of the contents of 

 the tube, was noted. In order to protect the observer from the 

 explosion, an inverted glass funnel was set over the whole, 

 part of the thermometer projecting beyond the tube of the fun- 

 nel. In carrying on similar experiments in the field of an 

 electro- magnet, more complete protection from the acid fumes 

 was obtained by means of the apparatus shown in the accom- 

 panying figure. The test-tube holding the iron and acid was 



■A 



i 



placed in a larger tube between the pole-pieces of the magnet. 

 Into the mouth of the larger tube there was fitted a thermome- 

 ter by means of a rubber cork. A long and narrow glass tube, 

 sealed into the outer tube just below the cork, afforded an exit 

 to the gaseous products of the reaction, which were caught over 

 a pneumatic trough. 



The electro-magnet was designed with special reference to 

 this work. The diameter of the cores was 55 mm , and the pole 

 pieces were so arranged that the tube containing the iron and 

 acid could be brought in a nearly uniform field of high inten- 

 sity. The magnet was actuated by a battery of " bichromate " 

 cells, and the current-strength regulated by a rheostat of simple 

 construction. The intensity of the magnetic field was esti- 



