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Show 88 DROSERA ROTUNDIFOLIA. CHAP. VI. a ppeai· s to be to a certain extent da nt.i sfe ptic., ash it checks the appearance of mould a~ In usona, t us preventing for a tiine the discoloration and decay of such substances as the white of an egg, ch esc, &c. It therefore acts like the gastric. juice of the higher ani1nals, which is known to arrest putrefaction by destroying the 1nicrozymes. As I was anxious to learn what acid the secretion contained, 445 ]eaves were washed in distilled water, given me by Prof. Frankland; but the secretion is so viscid that it is sc~r?el y possible to scrape or wash . off the wh?le. The conditiOns were also unfavourable, as It was late In the year and the ]eaves were srnall. Prof. Frankland with great kindness undertook to test the fluid thus collected. The leaves wore excited by clean partides of glass placed on them 24 hrs. previously. No doubt much more acid would have been secreted had the leaves been excited by animal matter, but this would have rendered the analysis more difficult. Prof. Frankland informs me that the fluid contained no trace of hydrochloric, sulphuric, tartaric, oxalic, or formic acids. This having been ascertained, tho 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 nurnl er ohta.inod, however, corresponded nearly with that of propionic acid; and I believe that this, or a mixture of acetic and lmtyric acids, were present in the liquid. The acid doubtless belongs to the acetic or fatty series." Prof. Frankland, as well as his assistant, oh erved (and this is an important fact) that the :fluid, "when aciditied with sulphuric acid, emitted a powerful odour like that of pepsin." The leaveH from which the secretion had been washed were also sent to Prof. Frankland; they were macerated for some hours, then acidi6ed 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. CHAP. 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 a~cortain whether acetic acid could be replaced, without the loRs of digestive power, by the allied aeids 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 containjng 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 corl'esponds 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 thermometer which indicated a temperature of 38° to 40° 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, |