Faraday to George Biddell Airy   23 April 1862

[Royal Institution embossed letterhead] | 23 April 1862

My dear Sir

When the electrochemical relations of two bodies are very nearly equal and because of changes in the surface state of the bodies very likely to be disturbed it is often impossible to tell why one gives way in a certain direction before the other¹. I have often known cases in which in the same medium the two bodies have alternated sometimes one getting a head of the other & sometimes falling behind it[.] Both well polished steel & blued steel (which is an oxidation) I have seen sometimes well preserved & sometimes easily decay[.]

You ask about prevention[.] I remember but only imperfectly that many years ago I was shewn the rusty state of a clock that had been fixed into an oak wood case. I believe it was South that shewed it to me[.] On trying the wood as to any power it might have of producing this action it was found that when holes were bored by a gimblet [sic] in the wood & little strips of delicate litmus paper were placed in the holes that paper was reddened in the course of a day or two by the acid vapour which rose from the wood. The corrosion of the iron parts of the clock was referred to this vapour. I think I tried & found that a like effect on the test paper was produced by some pieces of Mahogany & not by others[.]

Has this fact any bearing on the case of your chronometers & will it suggest a means of testing the wood of their boxes[.]

I am My dear Sir | Very Truly Yours | M. Faraday

G.B. Airy Esqr | &c &c &c