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Darwin Correspondence Project

To Edward Cresy   14 October [1860]1

15 Marine Parade, Eastbourne

Oct: 14th.

My dear Sir

I have received Hofman’s letter which is extremely interesting to me.2 It is a great shame to trouble you, but I should be very much obliged for a line here, of explanation, if you can give it.

Does Hofman mean that for instance 164,000 part3 of a grain of arsenic can be detected;—or that 14,000,000 part of a grain of iodine can be detected; surely that is quite incredible. Or does it mean that in a large body of water containing one part of iodine to 4,000,000 of water, that the starch in the course of time will out of this very weak solution absorb iodine so as to colour it. If this is meant these facts do not, in fact, show how little of these substances produce a visible chemical result.4

Pray forgive me, and believe me | Yours very truly | C. Darwin

At present I fully believe after endless experiments that 164,000 of a grain of C. of Ammonia produces on an absorbent gland of the Drosera a visible effect.5


Dated by the relationship to the preceding letter.
See the preceding letter.
The copyist originally wrote ‘1/64,800’, which has been corrected in an unidentified hand.
CD only resolved the point to his own satisfaction by performing further experiments on the sensitivity of Drosera leaves to chemical substances. His notes on these experiments are in DAR 60.1. His results were given in Insectivorous plants, chapters 7–9. With respect to ordinary chemical tests for salts, he reported that about 1/4,000 of a grain of arsenic could be detected, ‘but the power of detection depends much on the solutions under trial not being extremely weak’ (Insectivorous plants, p. 170 n.).
The figure given by CD is repeated in Insectivorous plants, p. 146, where CD described immersing four Drosera leaves in a weak solution of carbonate of ammonia for some hours. To find the amount of salt that each gland received, he divided the absolute quantity of ammonia in solution by the total number of glands. See also the first letter to Edward Cresy, 2 November [1860].


Insectivorous plants. By Charles Darwin. London: John Murray. 1875.


Discusses letter from A. W. v. Hofmann concerning solution of iodine in water.

Letter details

Letter no.
Charles Robert Darwin
Edward Cresy, Jr
Sent from
Source of text
DAR 143: 314
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 2950,” accessed on 29 September 2020,

Also published in The Correspondence of Charles Darwin, vol. 8