George Biddell Airy to Faraday   12 May 1854

Royal Observatory, Greenwich | 1854 May 12

Dear Sir

Since receiving your letter about Ozone¹, I have carefully examined into the circumstances of observation. The observers are so closely connected, and their habits are so completely formed one upon another, that I do not conceive that there is any risk of discordance from personal peculiarity. But another fact has come out which may have something to do with it, and concerning which I wish to learn whether it is recognized (it is new to me).

The fact is, that the tint of the paper sometimes goes off. In early morning it is sometimes deeply coloured, and by 9 o’clock the colour has nearly vanished. If at such a time a strip of the paper is torn off early and dipped into water, it gives a full purple: while that which remains, if treated in the same way at a later hour, shews no colour or very little.

I cannot say with certainty that this diminution of tint is different at different places, but I am inclined to think that it is.

I am, my dear Sir, | Yours very truly | G.B. Airy

Professor Faraday