330 Sir John F. W. Herschel on the Prepared 



If a piece of soft iron wire, well brightened, be immersed 

 in nitric acid of the density 1*399, the iron instantly becomes 

 brown, and an effervescence more or less lively takes place, 

 attended with the disengagement of red vapours ; but this 

 effervescence only lasts a few moments. It very soon ceases, 

 when the iron immediately recovers its metallic lustre, and 

 afterwards remains tranquil and intact at the bottom of the 

 acid for any length of time that may be desired. 



Iron thus treated, (which for the sake of brevity, I shall 

 in future call prepared iron,) may be withdrawn from the 

 acid and exposed to the air, or immersed in pure water or 

 in ammonia, without by these means regaining the property 

 of being attacked by nitric acid. In its prepared state it may 

 be touched (gently) either in the air or in acid, with gold, 

 silver, platina, mercury, glass, and several other substances 

 without destroying this state. But if its surface be rubbed 

 with force, so as to establish an intimate contact; if, for ex- 

 ample, it be scraped with the edge of a piece of glass, upon 

 a glass plate, its state of preparation is then destroyed, and if 

 it be again immersed in the acid, the effervescence followed 

 by total inaction again occurs, and the metallic lustre re- 

 appears : in a word, this is a complete renewal of the prepared 

 state. If, on the other hand, prepared iron be touched 

 either with copper, zinc, tin, bismuth, antimony, lead or iron 

 not prepared, when either in the air, water or acid, the pre- 

 pared state is destroyed, and the action of the acid commences 

 again with effervescence, &c, as usual. 



If a rather long piece of iron wire, prepared and moistened 

 with acid, be touched with copper at one of its extremities 

 while it is held suspended in the air over a glass plate, the 

 surface may be seen to become brown, not instantaneously, 

 and all over at once, but successively and by a movement, so 

 to speak, propagated (with rapidity certainly) from the ex- 

 tremity touched to the other extremity. When, in the pro- 

 gress of this embrowning, the limit of the brown colour reaches 

 a drop of acid suspended at an inflection of the wire, effer- 

 vescence and the complete decomposition of the drop of acid 

 take place. But if a wire immersed in the acid be thus 

 touched, the action is instantaneous throughout its whole 

 length. 



If these experiments be performed in a capsule containing 

 only a small quantity of acid, and many times repeated, the 

 acid becomes incapable of producing the prepared state in 

 the iron. This effect appears to arise partly from the heat 

 evolved, and partly from the presence of nitrous gas ; for 

 having impregnated pure acid with this gas until it acquired 



