EiaHTEENTH ANNUAL MEETING. 19 



of the magnet, even when the circuit was open. Upon repeating this experiment 

 with somewhat stronger acid we found the strength of field requisite to the destruc- 

 tion of passivity much greater with three parts of acid and one of water; the iron 

 remained passive under the residual magnetism of the magnet, and even in the field 

 produced by the current last employed. When the current was somewhat further 

 increased the iron became active. 



The tangent galvanometer when the transition took place, indicated .16 of the 

 current which in experiment (a) had been necessary to destroy the passivity in 

 concentrated acid at 51°. And the field was certainly of but little more than half the 

 strength of that under which, in experiment (/), iron had maintained its passivity 

 in the stronger acid 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 had reached .23 of that used in experi- 

 ment (a). 



(i) To determine in what strength of acid iron would become active at these 

 temperatures, without the influence of magnetism, or at least in the earth's field, 

 the usual quantity of iron was placed in concentrated acid and the acid rapidly di- 

 luted with water and with such small amounts of snow as were necessary to keep 

 the mixture in the neighborhood of 23°. Dilution was continued until the iron 

 had become active. The transition was much less clearly marked than at high tem- 

 perature or within the magnetic field, but repeated trials showed that the amount 

 of water to be added to 10 parts of acid in order to destroy passivity was about 13. 



(;■) To 20 cc. of concentrated acid at 0° containing passive iron, water was added, 

 with snow in quantities sufficient to prevent any marked rise of temperature. The 

 addition of 172.3 cc. of water and snow did not produce any noticeable reaction. 

 When placed in the field of the electro-magnet [strength about 20,000 H], efiferves- 

 cence began at once, and a dark-green solution of higher density than the acid was 

 formed. The color soon disappeared and a brown precipitate showed itself, which 

 was dark colored and highly magnetic. 



By repetition of the experiment outside of the field, it was found that a trace of 

 color could be detected when the acid had been diluted to \^, ( subsequent trials 

 changed this ratio to i,) and that the first evolution of gas could be noticed when 

 the strength of the acid had been further reduced to jj. Further dilution to ^-^^ 

 produced no further changes, nor did the evolution of gas at any time approach 

 that produced in the magnetic field at this temperature. 



It appears from the foregoing experiments, that the action upon the passivity 

 of iron in nitric acid consists in lowering the temperature of transition to the active 

 state, and that the intensity of the magnetic field necessary to convert passive into 

 active iron at a given temperature increases rapidly with the concentration of the 

 acid. The influence of magnetism upon the passivity of iron in other acids must 

 be determined by other experiments. The establishment of a satisfactory theory 

 of the influence of magnetism upon passivity will demand not only a much more 

 complete acquaintance with the phenomena touched upon in this paper, but a wider 

 knowledge than we now possess of the subject of chemical action in the magnetic 

 field. 



