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Darwin Correspondence Project

From J. S. Burdon Sanderson   25 April 1874

Note for Mr. Darwin April 25 18741

1. The purpose of the experiments was to determine the digestive activity of liquids containing pepsin, when acidulated with certain volatile acids belonging to the acetic acid Series with the same liquids acidulated with hydrochloric acid, in proportion similar to that in which it exists in gastric juice. The acids in question were Propionic Acid (Formula C6H6O11 [subscript numerals 6, 6, 11], Fusing Point 141o. C) Butyric Acid (Formula C8H8O4 [subscript numerals 8, 8, 4], Fusing Point 161oC) Valerianic Acid (Formula C10H10O4 [subscript numerals 10, 10, 4], Fusing Point 175o.C.).

The preparations of the acids were furnished me by Mr. Darwin.2

2. It has been determined empirically that the best results are obtained in artificial digestion when a liquid containing two per thousand of Hydrochloric acid gas by weight is used. This corresponds to about 6.25 cubic centimeters per litre of ordinary strong hydrochloric acid. The quantities of Propionic, Butyric and Valerianic acids respectively which are required to neutralise as much base as 6.25 cubic centimeters of HCl. are in grammes 4.04 of Propionic acid, 4.82 of Butyric acid, and 5.68 of Valerianic Acid. It was therefore judged expedient, in comparing the digestive powers of these acids with that of hydrochloric acid to use them in these proportions.

3. Five hundred cub. cent. of a liquid containing about 8 cub. cent. per Litre of a glycerin extract of the mucous membrane of the stomach of a dog killed during digestion, having been prepared, 10 c.c. of it were evaporated and dried at 110o. This quantity yielded 0.0031 of residue.

4. Of this liquid four quantities were taken which were severally acidulated with Hydrochloric, Propionic, Butyric and Valerianic Acids in the proportions above indicated. Each liquid was then placed in a tube which was allowed to float in a water bath containing a thermometer which indicated a temperature of 38o–40o. Cent. Into each, a quantity of unboiled fibrin was introduced, and the whole allowed to stand for four hours, the temperature being maintained during the whole time and care being taken that each contained throughout an excess of fibrin. At the end of the period each liquid was filtered. Of the filtrate, which of course contained as much of the fibrin as had been digested during the four hours, 10 c.c. were measured out, and evaporated and dried at 110o as before. The residues were respectively:—

In the liquid containing hydrochloric acid 0.4079
" " " propionic " 0.0601
" " " butyric " 0.1458
" " " valerianic " 0.1254

Hence, deducting from each of these residue left when the digestive liquid itself was evaporated, viz: 0.0031, we have

for propionic acid 0.0570
" butyric " 0.1437
" valerianic " 0.1223

as compared with 0.4048 for hydrochloric acid— these several numbers expressing the quantities of fibrin by weight digested in presence of equivalent quantities of the respective acids under identical conditions.

The results of the experiments may be stated thus:— If 100 represent the digestive power of a liquid containing pepsin with the usual proportion of hydrochloric acid, 14.0, 35.4 and 30.2 represent respectively the digestive powers of the three acids under investigation.

5. In a second experiment in which the procedure was in every respect the same excepting that all the tubes were plunged into the same water bath & the residues dried at 115oC the results were as follows:—

Quantity of fibrin dissolved in 4 hours by 10 cub. centims of liquid—

Propionic acid 0.0563
Butyric " 0.0835
Valerianic " 0.0615

The quantity digested by a similar liquid containing hydrochloric acid was 0.3376. Hence taking this as 100, the following numbers represent the relative quantities digested by the other acids

Propionic acid 16.5
Butyric " 24.7
Valerianic " 16.1

6 A third experiment of the same kind gave:—

Quantity of fibrin digested in 4 hours by 10. cc. of liquid

Hydrochloric acid 0.2915
Propionic " 0.1490
Butyric " 0.1044
Valerianic " 0.0520

Comparing, as before, each of the last of these three numbers with the first taken as 100, the digestive power of propionic acid is represented by

that of butyric acid by 35.8
& that of valerianic by 17.8

7. A further Experiment was made to ascertain whether the digestive activity of butyric acid (which was selected as being apparently the most efficacious) was relatively greater at ordinary temperatures than at the temperature of the body. It was found that whereas 10 c.c. of a liquid containing the ordinary proportion of hydrochloric acid digested 0.1311 gramme, a similar liquid prepared with butyric acid digested 0.0455 gramme of fibrin.

Hence, taking the quantity digested with hydrochloric acid at the temperature of the body as before as 100, we have the digestive power of Hydrochloric Acid at 16o–18oC. = 44.9 that of Butyric Acid at the same temp.— 15.6.

CD annotations

0.1 Note …1874] del ink
1.1 1. The purpose] right angle above and in left margin, red crayon
1.1 1. The purpose] after opening double quotes ink
1.4 juice.] before closing double quotes ink
1.4 The acids … Darwin. 2.1] crossed ink
1.5 Propionic Acid (Formula] below ‘*Butyric & [del ink] valerianic a’ ink
3.1 2. It has] after opening double quotes ink
3.4 hydrochloric acid.] right angle above and in left margin, red crayon
3.8 proportions.] before closing double quotes ink
4.1 3. Five] after opening double quotes ink
4.4 residue.] before closing double quotes ink
5.1 4. Of this] after opening double quotes ink
5.3 which was allowed] right angle above and in left margin, red crayon
6.3 for propionic acid 0.0570] right angle above and in left margin, red crayon
7.1 The results] after opening double quotes ink
7.4 investigation.] before closing double quotes ink
8.1 5. In a second] after opening double quotes ink
8.4 Quantity of fibrin] right angle above and in left margin, red crayon
9.6 16.1] before closing double quotes ink
10.1 6 A third] after opening double quotes ink
11.5 17.8] before closing double quotes ink, ‘[POINT HAND RIGHT SYMBOL]’ red crayon, ‘(See Back)’ ink ((see CD note, below)
12.1 7. A further] right angle above and in left margin, red crayon; after opening double quotes ink
13.3 15.6. before closing double quotes ink
Top of letter: ‘Insert Chapt. VI. p. 9 M.S.’ ink in box ink

CD note:

‘“The mean of these three sets of observations *(hydrochloric acid being taken as 100) [interl]

[em-dash above del ‘acid’]
gives for propionic 15.8
butyric 32.00
valerianic 21.4”

*So that the most powerful of these three acids [del]’


This note, with CD’s alterations, was reproduced in Insectivorous plants, pp. 89–91. Burdon Sanderson had been asked by CD to test the digestive properties of various acids believed possibly operative in Drosera (sundew; see Correspondence vol. 21, letter to J. S. Burdon Sanderson, 15 November [1873]). He gave a briefer report of the results in his letter of 30 March [1874], and CD had asked for further information on the temperature at which the experiments had been performed (letter to J. S. Burdon Sanderson, 31 March [1874].
CD had asked Edward Frankland to supply these acids to Burdon Sanderson (see letter to J. S. Burdon Sanderson, 15 November [1873]).


Correspondence: The correspondence of Charles Darwin. Edited by Frederick Burkhardt et al. 29 vols to date. Cambridge: Cambridge University Press. 1985–.

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


Purpose of experiments was to determine digestive activity of liquids containing pepsin. Gives required amounts of hydrochloric, propionic, butyric and valerianic acids. Describes experiment and gives results. Also experimented on digestive activity of butyric acid at greater temperatures than the termperature of the body.

Letter details

Letter no.
John Scott Burdon Sanderson, 1st baronet
Charles Robert Darwin
Source of text
DAR 58.2: 65–70
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 9427F,” accessed on 23 June 2024,

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