III.] INTRODUCTION. " liii 



be corroded in pits or holes. With imperfectly con- 

 ducting bodies the action is similar; at very short di- 

 stances they conduct^ and the action spreads or extends 

 from a point to the surrounding points. Sometimes^, in- 

 stead of forming a patch, the chemical action eats its 

 way in a dendritic form, pursuing the ramifications of 

 either the more oxidable portion of the substances or 

 of the more accessible negative points. The slightest 

 superficial change will produce a corresponding corro- 

 sion : thus, if you were to breathe on a polished plate of 

 iron and wipe away the condensed moisture from half 

 the plate, leaving that on the other half to evaporate, 

 the iron would subsequently rust in a dififerent manner 

 on the two parts. If you electrify a plate of glass having 

 letters cut in paper on it, and subsequently expose it 

 without the paper to hydrofluoric acid, the parts pre- 

 viously uncovered will be attacked ; and so, if you simply 

 allow the paper letters to remain on the ^lass for some 

 time (say a day or two), and then blow them off" by 

 breathing on the glass, or by the vapour of hydrofluoric 

 acid, the letters will be made manifest. You may easily 

 imagine a number of other instances. The efiects all pro- 

 ceed from a want of perfect homogeneity, either original 

 or impressed by some very trifling circumstance, and 

 from the fact that, points of action having once been 

 established, the corrosion is increased by the eff'ects it 

 itself produces and the deposits it forms. A dentist, to 

 cure a carious tooth, scrapes out all the diseased parts, 

 &c. Another cause of localized destructive agency is that 

 of crystallization : if a plate of common earthenware 

 has been used to contain saline solutions and is then 

 allowed to dry and is put aside, it will efi^loresce in spots 

 and a sort of vegetation will sprout up here and there, 

 disintegrating the plate in patches. I have now in my 



