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Darwin Correspondence Project

To Edward Frankland   12 April 1874

Down, | Beckenham, Kent.

Ap 12. 74

Dear Professor Frankland

I have just found out that the leaves of Drosera can digest all the phosphate of lime out of bones, & they remain clasped over them for an unusual length of time. Now I want much to ascertain whether it is the phosphate of lime or the animal matter in the bones which keeps them so long clasped. Could you therefore be so kind as to give me 2 or 3 grains of pure phosphate of lime with no animal matter or excess of acid. I believe I am right in supposing that it is phosphate and not super-phosphate in bone.1 I have been much delayed in my work on drosera by new editions of other books,—2 & therefore I shall have time & am very anxious to try an experiment suggested by you, namely whether the leaves would serve to test very weak sewage water.3 Somewhat later in the summer I will beg you to aid me in procuring a little of what may be considered weak sewage water, as I am quite ignorant what would be considered weak & how to obtain it. From your former kindness I think that you will excuse my troubling you.

Yours very sincerely | Ch. Darwin

P.S | The acids which you were so kind as to send to Burdon-Sanderson have proved of the greatest use to me, as he finds that all of them with pepsin digest animal matter.4


A form of calcium phosphate (hydroxylapatite, also known as ‘bone phosphate of lime’) is the main component of bones. Superphosphate is produced by adding sulphuric acid to calcium phosphate. CD described his experiments with phosphate of lime in Insectivorous plants, pp. 109–10.
CD had been working on Descent 2d ed. since 20 November 1873 (see Correspondence vol. 21, Appendix II); he was also working on Coral reefs 2d ed. (see letter to Smith, Elder & Co, 8 January 1874). CD published his work on Drosera, the sundew, in Insectivorous plants.
For CD’s experiment with sewage water, see letters to Edward Frankland, 14 May [1874] and 20 May [1874].
John Scott Burdon Sanderson had been helping CD to identify the acid in secretions of Drosera, and had received proprionic, butyric, and valerianic acid from Frankland (see Correspondence vol. 21, letter to J. S. Burdon Sanderson, 15 November [1873] and n. 2). Burdon Sanderson’s experiments with these acids are described in Insectivorous plants, pp. 89–91.


Finding that the leaves of Drosera digest all the phosphate of lime out of bones and then remain clasped over the bones for a long time, CD wants to determine whether it is the phosphate of lime or the animal matter in the bones that keeps them clasped. He asks EF to send 2 or 3 grams of pure phosphate of lime for his testing. [See 9411.] Will experiment in the summer using EF’s suggestion that leaves might serve to test weak sewage. Results of Sanderson’s experiments with acids of great use.

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. 9405A,” accessed on 15 September 2019,

Also published in The Correspondence of Charles Darwin, vol. 22