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Darwin Correspondence Project

To Francis Darwin   [before 15 April 1873]1


Dear Frank

I send straight to Pantlludw2 a bottle of formic acid & litmus paper (you can blue it with smelling salts) will you & Amy search some mountain tarns immediately & if you can find any not developed spawn, keep 2 basons in a warm room & add 6 drops of acid to one; & then even in a fortnight you ought to perceive a difference; & I dare say Amy wd observe for another week or two after you are gone— that is if the whole thing does not go squash3

Yours affectly | Ch. D.


The date is established by the relationship between this letter and the letter to Francis Darwin, 16 April [1873].
Pantlludw was the home of the Ruck family in Wales; Amy Ruck was Francis’s fiancée.
Litmus paper is used to test the acidity or alkalinity of materials. It is purple if neutral, pink if acid, and blue if alkaline. Smelling salts contain ammonium carbonate, an alkaline substance. The precise nature of CD’s experiment is not known, but CD was interested in the possibility that formic acid could prevent germination of seeds (see letter to J. T. Moggridge, 10 March 1873 and n. 1). He may have decided to extend the experiment to determine whether the acid would have a similar retarding effect on the spawn of aquatic animals.


Sends to Pantlludw [North Wales] bottle of formic acid. FD and Amy [Darwin] can search for spawn. If found, keep in two basins and add 6 drops of acid to one and look for differences.

Letter details

Letter no.
Charles Robert Darwin
Francis Darwin
Source of text
DAR 271.4: 6
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 8848A,” accessed on 28 September 2021,

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