1891.] The Passive State of Iron and Steel. 123 



the solution in the tube A shortly after commencement. Wben the 

 temperature of about 170 to 175 P. was reached a faint evolution of 

 gas in the form of bubbles was manifest, adhering to the steel, in the 

 warm tube only. No powerful solvent action or violent evolution of 

 nitric oxide gas, however, occurred in any of these experiments even 

 up to the temperature of 175 F., and these experiments were not 

 continued beyond this temperature. These results show that iron or 

 steel does not fully lose its passivity up to a temperature even of 

 175 F., though the passivity is shown to have been considerably 

 modified by temperature only. The critical point of temperature of 

 transition from the passive to the active state is therefore higher than 

 175 F., and is shown in the experiments of Part I, Series II, Table II, 

 to have been about 195 F. 



SERIES IV. 



The Passivity of Iron and various Steels increases with the Concentration 

 of the Nitric Acid. 



Schonbein considered that, " by immersing an iron wire in nitric 

 acid 1'50 sp. gr., it became likewise indifferent to the same acid of 

 T35 sp. gr.," and to all outward appearance this is so. 



Scheurer-Kestner considered that the passivity of iron was not de- 

 pendent on the greater or less degree of saturation of the acid. The 

 author, however, ascertained by the delicate electro-chemical mode of 

 experimentation employed, and hereafter referred to, that the pas- 

 sivity is materially influenced according to the concentration of the 

 nitric acid. 



The following experiments indicate that the property of passivity 

 in iron is not absolutely fixed or static, but that its passivity is modi- 

 fied to a certain extent in relation to the strength of the nitric acid 

 used. The general modus operandi was generally similar to that pre- 

 viously employed. Pairs of unmagnetised polished steel bars 6 inches 

 long, and 0'310 inch diameter, each pair be"ing of the same kind of 

 steel, and cut adjacently from one longer bar, were placed as before 

 in the wooden frame W, fig. 4, and then instantly and simultaneously 

 immersed in nitric acids, of two different degrees of concentration, 

 contained in the U'tbe arrangement, one limb of the U'^ 11 ^ 6 con- 

 taining red fuming nitric acid of sp. gr. 1'50, the other containing 

 nitric acid of sp. gr. T42, circuit being made through the galvano- 

 meter in the usual manner. The results, the average of repeated, 

 experiments in each case, are given on Table III, and show that the 

 passivity of iron increases considerably with the strength of the nitric 

 acid. 



VOL- XLIX. 



