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].
Experiments on the digestibility of certain preparations, sent by Mr. Darwin.
The preparations were as follows:—
1 `Hamoglobin'. This is a red substance insoluble in water, acids, alcohol, &c. It is therefore not hamoglobin, but probably haematin, 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 1.31
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 residence yielded nothing even when the digestion was prolonged indefinitely
Digestibility of Chlorophyll.
Various experiments made both with the preparation sent and with fresh chlorophyll shewed that this substance is entirely indigestible.
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.
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 after 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 Propazonic, 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 (propionc 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.
[Insert Chapt. VI. p. 9 m. 5]
(Note for Mr. Darwin April 25 1874
`1. The purpose of the following experiments was to determine the digestive activity of liquids containing pepsin, when acidulated with certain volatile acids belonging to the acetic Series in comparison with liquids acidulated with hydrochloric acid, in propotion similar to that in which it exists in gastric juice.'
`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 Liter 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 these 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 residues left when the digestive liquid itself was evaporated, viz. 0;0031, we have for
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 " —-- —- 0.8035
Valerianic " —-- —- 0.0615
The quantity digested by a similar liquid containing hydrochloric acid was 0.3376. Hence taking his 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
that of butyric and by 35.8
& that of vallerianic by 17.8 `` The mean of these three sets of
observations (hydrochloric acid being taken as 100)
gives for propionic -- 15.8
-- butyric -- 32.00
-- valerianic -- 21.4
So that the most powerful of there three acids
` 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 cc. 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 this 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.'