124 Mr. T. Andrews. [June 5, 



were immersed and rigidly supported in a large beaker of water on a 

 sand-bath, and the whole gradually heated to any required tempera- 

 ture, a thermometer, T, being suspended in the beaker. The mag- 

 netised and unmagnetised bars were in circuit with a delicate 

 galvanometer throughout each experiment. 



The arrangement is shown on fig. 2, and the results are given on 

 Table II. 



The nitric acid remained perfectly colourless until a temperature of 

 about 100 to 120 F. was reached, when a very pale-yellow tint 

 began to be perceptible in both- tubes, which gradually deepened 

 until the critical point of explosion was reached, at which the passivity 

 of the steel ceased. 



During the intervening period, the metal was acted upon but 

 slightly in both tubes ; a small evolution of gas showed itself in the 

 form of tiny bubbles adhering to the surface of the steel rod at a tempe- 

 rature of about 170 F. No extensive solvent action, however, occurred 

 until the acid arrived at a temperature of about 190 to 200 F. (the 

 explosion occurred most frequently at an average temperature of 

 195 F.), when, without any previous warning, a violent explosive 

 evolution of red nitric oxide gas took place, the steel being instantly 

 and vigorously attacked by the nitric acid. After the first explosion 

 the metal again became partially passive for a short time. The 

 explosion and solution of the steel first commenced in the limb 

 of the JJ -tube containing the magnetised steel bar, and the first inti- 

 mation of the passivity of the steel having been overcome was a 

 sudden and extensive fling of the galvanometer, positive for the 

 magnet bar. The bars were almost instantly removed from the 

 nitric acid; the solution in the limb of the (J-tube which had con- 

 tained the magnetised bar was of a very dark-brown colour compared 

 with solution in the other limb, thus giving further proof that the 

 magnetised bar had been first attacked. 



At the moment of explosion, the E.M.F. was very considerable ; in 

 several instances it varied from about y 5 th to as high as ^rd of a 

 volt, the magnetised bar being the electro-positive metal, see 

 Table II. 



In the experiment, Table II, col. 9, the temperature was not raised 

 above 185 F., and no explosive evolution of nitric oxide gas, there- 

 fore, occurred ; the metals were maintained in the nitric acid at a 

 temperature gradually reducing from 185 to 165 F. in course of 

 one hour. The steel did not fully lose its passivity, but was, how- 

 ever, only very slowly and partially acted upon by the nitric acid, 

 even at this high temperature. 



