254 PROTOPLASMIC ACTION AND NERVOUS ACTION 



the general nature of such processes.^ Passivity is 

 readily induced in an iron wire by immersion in strong 

 nitric acid (sp. gr. 1.42); the metal then remains un- 

 altered or chemically inactive when transferred to weak 

 acid (sp. gr. 1.2), unless it is artificially ''activated." 

 Activation may be induced by various means, chemical, 

 mechanical, and electrical. The following simple and 

 readily performed experiments will bring out clearly 

 the chief resemblances between the processes of activation 

 and transmission in the metallic systems and in Kving 

 protoplasm. 



When the passive iron wire is immersed in a dish of 

 nitric acid of about 60 volumes per cent concentration 

 (i.e., of commercial HNO3 c>f sp. gr. 1.42), no change 

 occurs, and the surface of the metal remains bright and 

 unaltered. If, however, it is then touched with a piece 

 of ordinary ''active" iron, or with a base metal Hke 

 zinc, a local reaction, accompanied by effervescence and 

 a darkening of the bright metalHc surface, is at once 

 initiated and sweeps rapidly over the whole wire from 

 end to end. In acid of the foregoing concentration, the 

 local reaction ceases in one or two seconds (at 20°), and 

 the metal reverts automatically to the passive state. 

 Immediately after this repassivation it is resistant to 

 activation and transmits the reaction imperfectly; on 

 standing, transmissivity gradually returns and within 

 a minute is usually again complete; the metal can then 

 be activated as before and the same phenomenon is 

 repeated. The passive wire may be activated mechani- 



^ For a general review of the phenomena of passivity in metals 

 cf. Bennett and Burnham, Journal of Physical Chemistry, XXI (1917), 

 107. 



