16 



KANSAS Academy of science. 



Table, showing influence of the duration of exposure to the acid upon the temperature at which iron 

 becomes active in HNO3 ; sp.gr. 1.368. 



4. On the influence of the presence of Iron-salts in the solution. 



The presence of even a small amount of the final products of the action of nitric 

 acid upon iron was found to lower the temperature of transition to the active state 

 to a marked extent. 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 boil- 

 ing in sulphuric acid. The failure to thoroughly cleanse the latter repeatedly reduced 

 the temperature 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 render iron passive, even at 20°. 



5. On the influence of the strength of acid. 



A much more obvious cause of variation in the temperature of transition than 

 those already alluded to, 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, showing temperature of transition from the passive to the active state in acid of different de- 

 grees of concentration. 



The time of exposure to the acid was in each case about 40 seconds. This deter- 

 mination agrees well with that noted by Schonbein for iron wire in nitric acid of 

 sp.gr. 1.35. He found the change to occur at 75°. 



III. 



In all these experiments, the method pursued had been very simple. The pow- 

 dered iron was placed in the bottom of a perfectly clean and dry test-tube, and the 

 acid, having been measured out into a beaker, was poured upon it. The test-tube 

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

 sition temperature, always indicated by an explosion of such violence as to expel the 

 greater portion of the contents from 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 funnel. Figure 1 shows the 

 arrangement of this apparatus. 



In carrying on similar experiments in the field of an electro-magnet, the com- 

 plete protection of the instrument from the acid fumes generated by the reaction, 

 was necessary. This was accomplished by the apparatus shown in figure 2, in which 

 all the fumes generated were carried away. The test-tube holding the iron and acid 

 for the reaction was placed in a slightly larger tube between the pole pieces of a 

 large electro-magnet. Into the mouth of the larger tube was fitted a thermometer 

 by means of a rubber cork; a long and narrow tube inserted just below the mouth 

 of this outer tube, afforded an exit for the gaseous products of the reaction. The 

 escaping gases were caught over a pneumatic trough. 



