74 DROSERA ROTUNDIFOLIA. [Chap. VI. 



rapour, 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 mo- 

 lecular weight of the acid. The number obtained, however, corre- 

 sponded 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." 



Professor Frankland, as well as his assistant, observed (and this 

 is an important fact) tliat 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 Professor 

 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 lit- 

 mus 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 ad- 

 visable 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 vale- 

 rianic. Dr. Burdon Sanderson was so kind as to make for me the 

 following experiments, the results of which are valuable, independ- 

 ently of the present inquiry. Professor 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 com- 

 parison with liquids acidulated with hydrochloric acid, in propor- 

 tion 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 vale- 

 rianic acids respectively which are required to neutralise as much 

 base as 6.25 cubic centimetres of HCl, are in grammes 4.04 of pro- 

 pionic acid, 4.82 of butyric acid, and 5.68 of valerianic acid. It was 

 therefore judged expe<lient, in comparing the digestive powers of 

 these acids with that of hydrochloric acid, to use them in these pro- 

 portions. 



" 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 liauid four quantities were taken which were sever- 

 ally acidulated with hydrocnloric, 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, con- 



