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Darwin Correspondence Project

From J. S. Burdon Sanderson   30 March [1874]1

49, Queen Anne Street. | W.

March 30

Dear Mr. Darwin,

I send you the results of our experiments.2

I attach much more importance to those relating to the three fatty acids than to the rest. They were very carefully made and yielded sharply defined results

They shew that each of these acids possesses the power of enabling pepsin to act on fibrin though in less degree than hydrochloric acid. If you make further experiments with the plants this summer, I should like to cheque them by comparison with pepsin digestion.

Very truly yours | J B Sanderson


Experiments on the digestibility of certain preparations, sent by Mr. Darwin.

The preparations were as follows:—

1 “Hæmoglobin”. This is a red substance insoluble in water, acids, alcohol, &c. It is therefore not hæmoglobin, but probably hæmatin, or a mixture of haematin with other bodies derived from blood.

2. “Globulin”. Probably a coagulated product from the crystalline lens.

3. “Mucin”. Probably acetic acid precipitate of some animal liquid containing mucin.

In order to test the digestibility of these substances each was subjected to the action of a digestive liquid containing 0.2 per cent of hydrochloric acid and about 1 per cent. of glycerin extract of stomach of dog.

It was first ascertained that this liquid was capable of digesting 1.31 of its weight of unboiled fibrin.

The results of the Experiments were as follows:

Percentage of fibrin digested

in one hour, as above stated

Percentage of “Haemoglobin” 0.456
" " “Globulin” 0.141
" " “Mucin” 0.296

It is to be observed that in each case the experiment consisted in subjecting an excess of the substance to be digested to the action of the digesting liquid. It is shown that each contains a certain proportion of digestible material. By other experiments it was ascertained with reference to the “haemoglobin” that the undigested residue yielded nothing even when the digestion was prolonged indefinitely3

Digestibility of Chlorophyll.

Various experiments made both with the preparation sent and with fresh chlorophyll shewed that this substance is entirely indigestible.4

Digestibility of recently precipitated globulin of serum of ox blood.

An experiment of the same kind as those above related with fresh globulin prepared by Schmidt’s method, yielded 0.865 per cent. to the digestive liquid after digestion for one hour at 38o C. It is therefore considerably less soluble than fibrin.5

Digestibility of Chondrin & Gelatin.

On this subject experiments have not been completed. I do not think however that it is possible to give any useful results, for neither gelatin nor chondrin can be obtained in a solid form otherwise than by the use of precipitants which alter their constitution in a degree which cannot be chemically defined. The result of this is that it is not possible to compare the action of the digestive liquid on products which stand in any definite relation to bodies actually existing in the organism.

Comparison of the action of Propionic, Butyric and Valerianic acids with that of Hydrochloric acid in digestion.

In these experiments a digestive liquid containing much less pepsin than was used in the former cases was prepared.

Exp. 1. Excess of unboiled fibrin was digested for four hours in a liquid containing 3.1 per ten thousand of glycerin extract, and 2 per thousand of hydrochloric acid.

Percentage of fibrin digested 4.079

Exp. 2. The same, with the exception that an equivalent proportion of propionic acid was substituted for hydrochloric.

Percentage of fibrin digested 0.563

Exp. 3. The same with butyric acid.

Percentage digested 0.835

Exp. 4. The same with Valerianic Acid—

Percentage digested 0.615.

Hence, the digestive powers of these acids as compared with that of hydrochloric acid are expressed by the numbers

16.5 (propionic acid)

24.7 (butyric acid)

16.1 (Valerianic acid)

the corresponding number for hydrochloric acid being 100.

There is therefore no relation between the order in which these acids occur in the series of fatty acids to which they belong and their digestive faculty.6

CD annotations

1.1 Experiments … indefinitely 9.5] crossed pencil
18.1 Exp. 1.] del pencil before pencil ‘An’
18.1 digested] before interl pencil ‘at a temperature of’
18.2 and 2] ‘and’ del pencil after interl pencil ‘of pepsin with’
25.1 Percentage digested 0.615.] ‘Replaced by better acid’ pencil
26.1 as compared with] del pencil
26.2 are expressed] after interl pencil ‘being taken as 100,’
26.6 the corresponding … 100.] del pencil; in margin pencil ‘Add exper at cooler Temperatures’
27.2 faculty.] point altered to semi-colon pencil before pencil ‘& although they are very inferior in power to [‘so that’ pencil del pencil] Hydrochloric acid, they are capable of digesting [altered from ‘digestion’] [cube] albumen [cooked]
Top of enclosure: ‘Chlorophyll’ pencil
Front of envelope: ‘Dr B. Sanderson On Digestion of Mucin Globulin &c with Hydrochloric Acid *& on Power of Acids [del pencil]’ blue crayon
Back of envelope:


The year is established by the relationship between this letter, the letter to J. S. Burdon Sanderson, 21 March 1874, and the letter from J. S. Burdon Sanderson, 23 March [1874].
Burdon Sanderson had undertaken a series of experiments for CD on the digestion of insectivorous plants (see Correspondence vol. 21, letter to J. S. Burdon Sanderson, 19 November [1873]).
CD reported these results in Insectivorous plants, pp. 120–1.
CD reported this result in Insectivorous plants, pp. 125–6.
CD reported this result in Insectivorous plants, p. 120 n.
These results, in expanded form (see letter from J. S. Burdon Sanderson, 25 April 1874), were included in Insectivorous plants, pp. 89–91.


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.


Sends results of experiments on digestion. Encloses two sets of notes: "Experiments on the digestibility of certain preparations sent by Mr Darwin" and "Note for Mr Darwin" [marked by CD for insertion in ch. 6 of Insectivorous plants].

Letter details

Letter no.
John Scott Burdon Sanderson, 1st baronet
Charles Robert Darwin
Sent from
London, Queen Anne St, 49
Source of text
University of British Columbia Library, Rare Books and Special Collections (Darwin - Burdon Sanderson letters RBSC-ARC-1731-1-28); DAR 58.2: 59–64
Physical description
ALS 3pp encl 12pp †

Please cite as

Darwin Correspondence Project, “Letter no. 9378A,” accessed on 28 March 2023,

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