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 141^o. C) Butyric Acid (Formula C8H8O4 [subscript numerals 8, 8, 4], Fusing Point 161^oC) Valerianic Acid (Formula C10H10O4 [subscript numerals 10, 10, 4], Fusing Point 175^o.C.).

The preparations of the acids were furnished me by M^r. 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 110^o. 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 38^o–40^o. 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 110^o as before. The residues were respectively:—

Hence, deducting from each of these residue left when the digestive liquid itself was evaporated, viz: 0^.0031, we have

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 115^oC the results were as follows:—

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

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

6 A third experiment of the same kind gave:—

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

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

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 16^o–18^oC. = 44^.9 that of Butyric Acid at the same temp.— 15^.6.