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Darwin Correspondence Project

To Edward Frankland   13 October 1873

Down, | Beckenham, Kent.

Oct 13. 1873

My dear Sir

I am quite ashamed of myself at the trouble which I have caused you, more especially considering the value of your time. I did not in the least know what labour it was to test for organic acids.1 I can only say that I feel very grateful. The negative information is very interesting to me, for to the day of my death I shd have been haunted with the belief that the acid was hydrochloric.2 It will be extremely useful to me to give your approximate determination of the acids. There can be no doubt that the viscid secretion, whenever the leaves were closely clasped over any object, instantly coloured my litmus paper bright red, for I have tried this many scores of times.3 When a substance like Casein or Chlorophyl is on a leaf, which is not readily absorbed, but excites much secretion, I have found that it has strongly coloured litmus paper for 8 successive days. Although I washed so many leaves & gently scraped them in the Sol of C. of Soda,4 yet a large quantity of the secretion still adhered to them, & this may have contained a good deal of the acid.

Unfortunately, before sending you the leaves, I partially dried them between blotting paper.

In my ignorance I am perplexed about the litmus paper which you have so kindly sent me.5 It is, as you may see by the enclosed, of a pink colour, but becomes blue if held in the vapour of Carb. of Ammonia. There certainly is no acid vapour in my room; & if the pink colour is not right, I imagine that there must have been a bottle with some volatile acid in the Post bag with your letter; but pray do not give yourself any more trouble about it.

With my cordial thanks | yours very sincerely | Charles Darwin


CD had asked Frankland to analyse the acid secretion of Drosera; see letter from Edward Frankland, 10 October 1873 and n. 1.
Frankland had not been able to discover what acid was in the secretion of Drosera, but had ruled out five possibilities, including hydrochloric. The digestive secretions of animals with stomachs contain hydrochloric acid.
Litmus paper is turned red by acids.
Frankland had advised CD to wash the leaves of Drosera in a solution of carbonate of soda (letter from Edward Frankland, 15 July 1873).


Finds the negative information sent by EF of great interest [see 9094].

More on his own experiments and the perplexing results when using the sensitive litmus paper.

Letter details

Letter no.
Charles Robert Darwin
Edward Frankland
Sent from
Source of text
The John Rylands Library, The University of Manchester
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 9096A,” accessed on 27 October 2021,

Also published in The Correspondence of Charles Darwin, vol. 21