412 



TENTH ANNUAL REPORT OF 



over a spirit-lamp, are not attacked either by concentrated or dilute 

 nitric acid." 



These experiments may he made much more conveniently with iron 

 wires. An iron wire placed in nitric acid of the specific gravity of 

 1.5, becomes passive ; and it assumes the same condition if heated in 

 a spirit- flame to iridescence. The wire thus rendered passive can 

 then be dipped in dilute acid without being attacked, while an ordinary 

 iron wire would occasion a violent liberation of gas. The dilution of 

 the acid cannot exceed a certain limit, which as yet is not ascertained. 

 Schonbein has determined that nitric acid of 1.36 specific gravity, 

 diluted with 15 and more volumes of water, attacked heated iron wire 

 as it does ordinary wire. 



By exposing an iron wire to nitric acid of the spec. grav. 1.35, it 

 will be attacked with great violence. On removing the wire from 

 the acid after a second, and holding it a few moments in the air, and 

 then returning it to the liquid, the action of the acid on the iron will 

 be perceptibly weaker. After three or four alternate immersions and 

 removals, a tolerably slow action appears ; and, at the fifth, or, at 

 the latest, at the sixth immersion, absolute chemical indifference 

 takes place, exhibited in the perfect metallic lustre of the surface of 

 the wire thus treated, which generally characterizes the iron ren- 

 dered passive in nitric acid. 



From these facts, there does not appear to be the least relation be- 

 tween the passivity of iron and its electrical properties ; but that such 

 Fig. 36. relations do exist may be shown by the following 



method of inducing the condition. 



First dip in nitric acid, of the spec. grav. 1.35, 

 a platinum wire P, Fig. 36; touch it with a well- 

 cleaned iron wire, and the latter wire will not be 

 attacked by the acid, when immersed, so long as it 

 remains in contact with the platinum wire, altliough 

 the same wire alone would be at once attacked by 

 the acid. 



If, instead of the platinum wire, the iridized end 

 of an iron wire, thus rendered passive, be immersed 

 in the liquid, it will play the part of the platinum 

 wire^ in the above experiment, perfectly. Fig. 37 

 represents a variation of this experiment. The iri- 

 dized and hence passive end of an iron wire is im- 

 mersed in nitric acid of the spec. grav. 1.35, and is not attacked. Now 

 bend it so that the end E, which was not heated, dips 

 into the acid. No action takes place ; but if the end 

 E be placed in the acid without P, violent action will 

 occur. 



It should be added that these phenomena no longer 

 appear wdien the temperature of the acid is raised to 

 8U°, and that they are the weaker, the nearer the 

 acid is to this degree of temperature. 



If the wire E, Fig. 36, be thrust into the liquid 

 while it is in contact with P, the latter may be al- 

 together removed without E losing its passivity ; in- 

 deed, with the wire E thus rendered passive, the same 



Fig. 37 



