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Darwin Correspondence Project

From T. L. Brunton   23 May 1874

23 Somerset St. Portman Sq | London W.

May 23d. 1874

My dear Sir

I am ashamed to see the date on this & to compare it with that upon yours1 most especially as my excuse for delay is insufficient when I come to look fairly at it although when hazily regarded in the way one sometimes looks at such things it seemed to me to assume larger proportions than it does now when I bring it into focus. I don’t know indeed how to present it, so insignificant does it look without the hazy medium which I can’t give but the whole is that finding one of my reagent bottles empty I wanted to replenish it from my stock at the Hospital2 & regularly forgot to replenish while there till so many days had passed that I thought I must needs give you the whole answer to your other questions as well as that regarding the milk in my reply but unfortunately find I can’t do so till some day next week & so I write now as I am thoroughly ashamed of my delay.3 The part of the slide nearest the label & which seemed to you the best contained small refracting bodies which were either oil or resin. They were soluble in ether but so was the resin also with which the glass was secured   I suppose however that they presented much the same appearance before the balsam was put on & therefore think that one may say with confidence that they were oil. In the other part of the slide were several bacteria. They seemed to refract light more strongly than usual but Dr. Ferrier who worked at the subject of low organisms for several months felt almost certain that they were bacteria.4

It is very odd that adding a minute drop of hydrochloric acid of 1 per cent should stop the digestion of albumen while a drop of 12 per cent should not. Too strong an acid certainly stops the digestive action of pepsin but the strength of 1 per cent (of ordinary acid = ·318 of real acid) is not very far from that which Brucke found to digest albumen most actively.5 For fibrin the best strength was 2·7 & 2·76 per cent of commercial acid when raised to 4· per cent digestion became much slower. When the acid was diluted digestion became gradually slower to 1·38 & 1·41 & when the dilution had reached ·69 digestion had become considerably slower. The best strength for neutralized white of egg was between 3·77 & 5. For not neutralized albumen 5·46. These numbers are taken from the abstract of Brücke’s paper in Canstatts Jahresbericht & I have converted the amount of absolute ClH in a litre which he gives into commercial acid per cent.6

Commercial pepsin consists of the cells lining the stomach scraped off with a blunt knife & dried & contains a good deal of albuminous material. I don’t think I have ever got it quite dissolved in any of my experiments but I have not yet ascertained even approximately what proportion remains undissolved I hope to be able in a few days to give you the results of the digestion of bones & of urea & shortly after some facts about digestion by the papaw7

I remain my dear Sir | Yours very truly | T Lauder Brunton

CD annotations

1.1 I … delay. 1.11] crossed blue crayon
1.11 The] opening square bracket blue crayon
1.11 The … resin. 1.13] ‘Milk’ added blue crayon
1.14 I … oil. 1.16] ‘(Yes)’ added blue crayon
1.16 In … were bacteria. 1.19] crossed blue crayon
2.1 It is … should not. 2.2] ‘[Probably] leaf injured’ added blue crayon
2.3 (of … actively. 2.5] double scored blue crayon
2.5 For … cent. 2.12] crossed blue crayon
3.6 papaw] underl blue crayon
Top of letter: ‘Casein of Milk | Acid not too strong for digestion’ ink


See Correspondence vol. 22, letter to T. L. Brunton, 11 May 1874.
Brunton was a physician at St Bartholomew’s Hospital, London.
CD had asked Brunton to analyse a microscope slide on which he had smeared the remnants of skimmed milk that had been placed on the glands of Drosera (sundew) for several hours, and to answer other questions concerning the digestive processes of insectivorous plants (see Correspondence vol. 22, letter to T. L. Brunton, 11 May 1874). CD had made the experiment to ascertain whether the digestive secretion would dissolve casein in milk (Insectivorous plants, p. 114).
David Ferrier had published an article on the bacterial species Sarcina ventriculi in 1872 (Ferrier 1872).
Ernst Wilhelm von Brücke. The concept of a ‘real acid’, that is, a hypothetical acid stripped of its water content, was developed in the late eighteenth century by Richard Kirwan (see Taylor 2008 for more on Kirwan’s theory and its reception). Kirwan produced tables for determining the amount of real acid in any concentration of acid, given a known specific gravity (Kirwan 1797).
Brunton obtained his information from the abstract of Brücke 1861 that appeared in Canstatt’s Jahresbericht über die Fortschritte der Gesammten Medicin in allen Ländern 5 (1862): 74–5. ‘Absolute ClH’ probably refers to the amount of chloride of hydrogen in aqueous solution.
Brunton described the digestive action of the ferment papain, derived from the pawpaw fruit (Carica papaya) in Brunton 1885, pp. 777–8.


Brücke, Ernst Wilhelm. 1861. Beiträge zur Lehre von der Verdauung. Sitzungsberichte der Akademie der Wissenschaften in Wien. Mathematisch-naturwissenschaftliche Klasse 43 (2d part): 601–23.

Brunton, Thomas Lauder. 1885. A text-book of pharmacology, therapeutics and materia medica; adapted to the United States pharmacopœia. London: Macmillan & Co.

Ferrier, David. 1872. The constant occurrence of Sarcina ventriculi (Goodsir) in the blood of man and the lower animals: with remarks on the nature of sarcinous vomiting. British Medical Journal, 27 January 1872, pp. 98–9.

Kirwan, Richard. 1797. Additional observations on the proportion of real acid in the three antient known mineral acids, and on the ingredients in various neutral salts and other compounds. [Read 16 December 1797.] Transactions of the Royal Irish Academy 7 (1800): 163–297.


Comments on his examination of slides [of milk casein?] sent by CD.

Surprised by CD’s finding that a drop of one per cent hydrochloric acid stops digestion of albumen by Drosera.

Letter details

Letter no.
Thomas Lauder Brunton, 1st baronet
Charles Robert Darwin
Sent from
London, Somerset St, 23
Source of text
DAR 58.1: 120–2
Physical description
ALS 6pp †

Please cite as

Darwin Correspondence Project, “Letter no. 10512,” accessed on 1 June 2023,

Also published in The Correspondence of Charles Darwin, vol. 24 (Supplement)