T4 DROSERA ROTUNDIFOLIA. [Cuap. VI. 
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. 
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 ex- 
periments, 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 corre- 
sponds 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 prepared, 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, 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 ther- 
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