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Darwin Correspondence Project

To Edward Frankland   8 February 1879

Down, | Beckenham, Kent. | (Railway Station | Orpington. S.E.R.)

Feb. 8th 1879

My dear Dr. Frankland

The case stands thus: we find that drops of water left standing for some days on certain leaves injured them, & afterwards my son Francis found that water left on leaves, or leaves immersed in water (with their stalks outside) in most cases made the water alkaline.1 When such was boiled down, the residue heated to redness & then redissolved, the solution was still alkaline. This shows that it cannot be Ammonia. There is a wide difference in the degree of alkalinity when different species of plants are tried; & if I remember rightly none in a few cases; but my son is away from home at present.—2 What we are very anxious to learn is what is the alkali in the water; & it is exceedingly kind of you to have it analysed.— I presume that it will be impossible to give the quantity of alkali.— As yet we have found no notice of any such phenomenon having been observed, excepting that Sachs says that dew on grass is alkaline, but he did not ascertain whether this was due to ammonia.—3 We suspect, naturally, potash, & it seems to us a strange thing if a substance which is considered so valuable a manure is secreted by the leaves.— I shd. have said that the water becomes alkaline when the plants are kept in the dark.—

With very true thanks | Yours sincerely | Ch. Darwin


CD had requested pure water from Frankland to try these experiments on the exudation of alkaline matter from leaves on growing plants; see letter to Edward Frankland, 4 January [1879]. Neither Francis Darwin nor CD published on this subject.
Francis Darwin left for a month-long visit to Algiers on 4 February 1879 (Emma Darwin’s diary (DAR 242)). The substance in the residue could not be ammonia (NH3) because ammonia becomes a gas at room temperature. When the solution was boiled down or boiled dry, some trace ammonia salt would remain in the residue unless the water was very pure, although probably not sufficient to cause great alkalinity.
Julius Sachs had suggested the experiment to Francis; he had supposed the alkalinity of the dew was caused by the exudation of potash; see Correspondence vol. 26, letter from Francis Darwin, [4–7 August 1878] and n. 5.


Gives results of the experiments: the leaves in most cases make the water alkaline. It cannot be ammonia. He and his son, Francis, suspect potash, which is valued as a manure. Will be grateful for the analysis EF has offered.

Letter details

Letter no.
Charles Robert Darwin
Edward Frankland
Sent from
Source of text
The John Rylands Library, The University of Manchester (Frankland Collection)
Physical description
ALS 4pp

Please cite as

Darwin Correspondence Project, “Letter no. 11864A,” accessed on 6 July 2022,