Cuar. VI. DIGESTION. 89 
Although it has long been known that pepsin with acetic 
acid has the power of digesting albuminous compounds, 
it appeared advisable to ascertain whether acetic acid could 
be replaced, without the loss of digestive power, by the 
allied acids which are believed to occur in the secretion 
of Drosera, namely, propionic, butyric, or valerianic. Dr. 
Burdon Sanderson was so kind as to make for me the follow- 
ing experiments, the results of which are valuable, indepen- 
dently of the present inquiry. Prof. Frankland supplied the 
acids. 
“1. The purpose of the following experiments was to deter- 
mine 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 proportion similar to that in which it exists in gastric 
juice. 
«92. 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 centimetres 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 centimetres 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. of a glycerine extract of the mucous membrane of 
the stomach of a dog killed during digestion having been pre- 
pared, 10 cub. cent. of it were evaporated and dried at 110°. 
This quantity yielded 0:0031 of residue. 
“4. Of this liquid four quantities were taken which were 
severally acidulated with hydrochloric, propivnic, 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° to 40° Cent. Into each, a quantity of un- 
boiled 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 through- 
out 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, 
