Chap. VI. DIGESTION. 89 



Althougli 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. 



" 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 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, 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 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, 



