trace of hydrochloric, sulphuric, tartaric, oxalic, or formic acids.
This having been ascertained, the remainder of the fluid was evaporated
nearly to dryness, and acidified with sulphuric acid; it then evolved
volatile acid vapour, which was condensed and digested with carbonate
of silver. The weight of the silver salt thus produced was only .37
gr., much too small a quantity for the accurate determination of the
molecular weight of the acid. The number obtained, however,
corresponded nearly with that of propionic acid; and I believe that
this, or a mixture of acetic and butyric acids, were present in the
liquid. The acid doubtless belongs to the acetic or fatty series.
Prof. Frankland, as well as his assistant, observed (and this is an
important fact) that the fluid, when acidified with sulphuric acid,
emitted a powerful odour like that of pepsin. The leaves from which
the secretion had been washed were also sent to Prof. Frankland; they
were macerated for some hours, then acidified with sulphuric acid and
distilled, but no acid passed over. Therefore the acid which fresh
leaves contain, as shown by their discolouring litmus paper when
crushed, must be of a different nature from that present in the
secretion. Nor was any odour of pepsin emitted by them. [page 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 following
experiments, the results of which are valuable, independently of the
present inquiry. Prof. Frankland supplied the acids.
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 proportion 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 centimetres per litre of ordinary strong
78