# To F. C. Donders   7 July 1874

Down. | Beckenham Kent.

July 7— 74—

My dear Prof Donders.

My son George writes to me that he has seen you, & that you have been very kind to him for which I return you my cordial thanks.—1 He tells me on your authority— of a fact which interests me in the highest degree, & which I much wish to be allowed to quote. It relates to the action of one millionth of a grain of Atropine on the eye.— Now will you be so kind whenever you can find a little leisure to tell me whether you yourself have observed this fact, or believe it on good authority— I also wish to know what proportion by weight the atropine bore to the water of solution, & how much of the solution, was applied to the eye— The reason why I am so anxious on this head is that it gives some support to certain facts repeatedly observed by me—with respect to the Action of Phosphate of Ammonia on Drosera— The $\frac{1}{4,000,000}$ of a grain absorbed by a gland clearly makes the tentacle which bears this gland become inflected; & I am fully convinced that $\frac{1}{20,000,000}$ of a grain of the Crystallized salt. (i  e containing about $\frac{1}{3}$ of its weight of water of crystallization) does the same.2 Now I am quite unhappy at the thought of having to publish such a statement   It will be of great value to me to be able to give any analogous facts in support.3 The case of Drosera is all the more interesting as the absorption of the salt or any other stimulant applied to the gland causes it to transmit a motor influence to the base of the tentacle which bears the gland—

Pray forgive me for troubling you, & do not trouble yourself to answer this until your health is fully reestablished—

Pray believe me. | your’s very sincerely— | Charles Darwin.

## Footnotes

George Howard Darwin had met Donders in Utrecht while travelling in Europe (letter from Emma Darwin to Leonard Darwin, 7 July 1874 (DAR 239.23: 1.19)).
Water of crystallisation is the water that is incorporated into the crystal lattice structure of many compounds. In Insectivorous plants, p. 155, CD noted that the crystallised phosphate of ammonia that he used contained 35.33 per cent water of crystallisation.
In Insectivorous plants, p. 173, CD cited Donders for information on the paralytic action of one millionth of a grain of highly diluted sulphate of atropine on the eye muscles.

## Summary

Asks about the effect of atropine on the eye. Is interested in parallel case: influence of phosphate of ammonia on glands of Drosera.

## Letter details

Letter no.
DCP-LETT-9535
From
Charles Robert Darwin
To
Frans Cornelis (Franciscus Cornelius) Donders
Sent from
Down
Source of text
DAR 143: 417
Physical description
4pp