HANDBOOK FOE BIO-CHEMICAL LABORATORY. 31 



are laevo-rotatory, the specific rotatory power being (a) D = 

 - 31.6. 



Ptyalin, 



Preparation. 1. Mixed human saliva is strongly acidulated 

 with phosphoric acid and the mixture carefully neutralized by 

 the careful addition of lime-water, which produces a copious 

 precipitate of calcium phosphate, and which carries down with 

 it a large proportion of the proteids present, together with all 

 the ptyaliu. On extracting the precipitate, after filtration, 

 with a Yolume of water equal to that of the saliva originally 

 employed, the enzyme passes into solution and is obtained 

 therefrom by the addition of absolute alcohol, which precipi- 

 tates it in the form of white flocculi, which when dried in 

 vacuo appears as a white powder. (Cohnheim.) 



2. Saliva is diluted with an equal volume of water and 

 saturated with ammonium sulphate. The precipitate thus 

 formed is treated on the filter for 5 minutes with strong al- 

 cohol, removed from the filter, and further treated with ab- 

 solute alcohol for 1-2 days. It is now dried at 30 C. and 

 yields, on extraction with a volume of water equal to that of 

 the original saliva, a solution which is actively zymolytic and 

 free from proteids. (Krawkow.) 



Pepsin. 



Preparation. 1. The mucous coat of the pig's stomach is 

 scraped with a watch-glass, and after carefully washing and 

 drying, by pressing between filter-paper, it is finely minced. 

 The mass is now digested with a 5$ solution of H 3 P0 4 at 35 C. 

 until nearly all is dissolved. Nearly neutralize the filtered 

 solution with lime-water, which causes a precipitate of calcium 

 phosphate, and which carries down with it the greater part of 

 the pepsin previously in solution. The peptones and para- 

 peptones remain in solution. The gelatinous precipitate is 

 carefully washed with water, pressed between filter-paper, 



