To Edward Frankland   14 May [1874]¹

My dear Prof. Frankland

I find that I have leaves enough to try Sewage water.² Will you therefore be so kind as to send me some: of such strength as you w^d. feel some surprise at being detected by any one except by a chemist. Perhaps the best plan would be to send only 1 or 2 ounces, & this c^d be sent in bottle in little box by post, which w^d. save much delay & putrification, as Railway to here is slow.— You could say whether I ought to add twice or thrice as much water to what you send, so as to get the proper dilution.

If what you consider to be weak should act, I could then still further dilute it. I sh^d use only 1 dr or $\frac{1}{2}$ dr. to each leaf. so very little is wanted. If the fluid contains Phosphate of Ammonia it will certainly act, & almost certainly if it contains the Nitrate of ammonia.—³

In next Nature you will see that I have used your information, & quoted your excellent illustration.⁴

Yours very sincerely | Ch. Darwin

P.S. If by any chance you have any Oleic Acid in your Laboratory, will you send 3 or 4 gr. if solid or a few drops if liquid. But not worth preparing on purpose   I now suspect that this is cause of Olive oil acting on Drosera; many acids, even such as Benzoic acid, are astonishingly powerful poisons on Drosera & cause inflection; some few acids have no such effect. Acetic acid is powerful.—⁵