24 ORGANIC CONSTITUENTS. 



Pepsin is thrown down from its aqueous solution by bichloride 

 of mercury, acetate of lead, the sulphates of iron, sulphate of 

 copper, and perchloride of tin. Ferrocyanide of potassium 

 throws down no precipitate from an acidulated solution of pepsin. 



Pepsin, which is precipitated from a concentrated aqueous 

 solution by anhydrous alcohol, is said to lose its digestive power. 



According to Liebig, pepsin as a distinct compound does not 

 exist ; he ascribes the solvent power of the gastric juice to the 

 gradual decomposition of a matter dissolved from the mem- 

 brane, aided by the oxygen introduced in the saliva. (Animal 

 Chemistry, p. 109 et seq.) 



Diagnosis. Pepsin is soluble in water, insoluble in absolute 

 alcohol and ether; it is known by its precipitation by dilute 

 acids, by the precipitate being redissolved in a sHght excess of 

 the test, and by the non-occurrence of a precipitate on the ad- 

 dition of ferrocyanide of potassium to the acid solution. It is 

 fui'ther distinguished from albumen by its being precipitable by 

 acetic and dilute hydi'ochloric acids. 



6. Ptyalin. This term has been applied to a peculiar animal 

 matter that exists in the saliva. The following is the best me- 

 thod of obtaining it. Fresh saliva must be neutralized with 

 acetic acid, and then evaporated on the water-bath ; the residue 

 must be extracted first with alcohol, i and then Avith spirit. The 

 ptyalin will remain undissolved amongst the protein-com- 

 pounds, and must be extracted from them by the addition of 

 water, in which it is readily soluble, and with which it forms a 

 viscid fluid. The evaporation of this aqueous solution yields 

 ptyalin free from all animal matters, but containing a trace of 

 salts. When dry it is colourless, transparent, and brittle, devoid 

 of odour, but with rather a sickly taste. 



It is readily soluble in water, but is insoluble in alcohol and 

 ether. It is precipitated from its aqueous solution by alcohol, 

 but not by the mineral acids, met.allic salts, acetic or tannic 

 acid. 



Our knowledge of this substance is by no means accm*ate ; 

 no analysis has ever been published, and there is no doubt that 



' The term spirit is used to denote alcoliul of spec. giav. 833, wluch contains 

 about 85;^ of anhydrous alcohol ; 1)\ akuhol, anhydrous alcohol of si)cc. grav. -792 is 

 imiilicd. 



