relation to Time and Temperature. 4G3 



and a metallic unit, represented by a superficial layer of the metal 

 plaque under examination, one milligram in weight and one square 

 centimetre in area. 



As regards the metals iron and nickel, which were obtained in 

 the thin sheets 01 mm. in thickness, each sheet was cut into 

 strips one centimetre wide, and a strip was again subdivided into 

 discs of metal one or more centimetres in length. A square disc 

 therefore had a surface area of two square centimetres, for in this 

 case the edges of a disc were not considered. In the thicker 

 cobalt and copper sheeting, however, an allowance was made for 

 the surface area of the four edges ; and a 2-centimetre disc was 

 modified in shape accordingly. The nitric acid used was the 

 ordinary pharmacopoeial fuming acid. 



In the course of the investigation it was found that each 

 of the three metals under observation had a critical point in the 

 temperature-scale, above which its passivity ceased. The change 

 was an abrupt one ; and the temperature, at which this took 

 place, differed greatly for each metal. Iron for instance did not 

 lose its passivity under favourable circumstances until the boiling- 

 point of water was nearly reached, when the change occurred with 

 almost explosive violence. Cobalt, on the other hand, lost its 

 passivity at or about 50° F. Nickel occupied an intermediate 

 position. The critical temperature of sheet-nickel was about 

 175° F. ; whilst in Mond nickel it was some ten degrees higher. 

 The samples of nickel differed in another respect from the two 

 preceding metals ; the change from a passive to an active state 

 was in their case more gradual than in either iron or cobalt. It 

 therefore became necessary to define the passive state more clearly 

 than heretofore. This has been provisionally done as follows : — 

 Sheet iron, nickel, and cobalt, when immersed in fuming ■ nitric 

 acid, may be considered to assume a passive condition if after 

 ten seconds' exposure to the acid a given metallic unit of a sample 

 is not dissolved. The critical temperature of copper, which I 

 believe is not usually supposed to exhibit passivity, is probably 

 a few degrees above that of cobalt. 



Immediately below the critical point there exists for each 

 of the three metals a temperature-zone of some twenty degrees 

 (or even more in the case of sheet iron) wherein the phenomenon 

 of passivity may or may not occur under apparently identical 

 conditions. The samples of rolled cobalt showed this peculiarity 

 markedly. It is by no means unusual for two out of three 

 specimens of this metal, immersed in the acid at 40° F., to exhibit 

 passivity while the remaining sample shows the signs of intense 

 activity. Within its proper zone of instability — to so name it — 

 each metal is therefore liable to undergo an abnormal crisis at 

 any given temperature. If however the metal is placed in the 



