CHEMICAL BASIS OF THE ANIMAL BODY. 55 



aqueous or other solution by saturation with neutral ammonium 

 sulphate 1 . They are conveniently soluble in glycerine 2 from which 

 they may as before be precipitated by an excess of alcohol. None of 

 the enzymes are diffusible and hence they may readily be freed from 

 any admixed diffusible substances by means of dialysis 3 . They possess 

 further the remarkable property of adhering with great tenacity to 

 any finely divided precipitate which is formed in the solutions in 

 which they are present, more particularly if the precipitate is of a 

 viscid or gelatinous nature 4 . It is not however possible to base upon 

 the above properties any general method of preparing the enzymes 

 which is equally applicable to each of them ; some are most readily 

 prepared in a fairly pure state by one method, some by another, and 

 very many by the conjoined application of two methods. A further 

 consideration must not be lost sight of in connection with the separation 

 of the enzymes from the parent tissues ; this is the fact that in some 

 cases the enzymes do not exist in the free and active conditions in the 

 cells of the respective tissues but in the form of an inactive antecedent 

 to which the name of 'zymogen' is usually applied 5 . Hence to obtain 

 an active extract it is frequently necessary to treat the tissue with 

 some such reagent as shall ensure the conversion of the zymogen into 

 the active enzyme. 



During prolonged digestions it is essential to insure the absence of 

 any changes due to the development of bacteria or other organisms. 

 The most suitable antiseptics for this purpose are salicylic acid 

 (*1 p.c.) and thymol ('5 p.c.). These reagents are dissolved in a small 

 quantity of alcohol and added in the above proportions to the digestive 

 mixture. 



It is frequently a matter of the utmost importance to determine 

 whether the hydrolytic power of any given preparation is due to the 

 action of a soluble enzyme or of a ferment (organised). The dis- 

 crimination is most readily effected by carrying on the digestion in 

 presence of chloroform, which is inert towards the enzymes but inhibits 

 the activity of ferment organisms 6 . 



1 Kiihne, Verhand. d. naturh.-med. Ver. Heidelb. m. 1886, S. 463. Also 

 Centralb. f. d. med. Wiss. 1886, Nr. 45. Krawkow (Eussian). See Per. d. deutsch. 

 chem. Gesell. Referatband. 1887, S. 735 or Maly's Jahresber. xvn. S. 466. 



2 v. Wittich, Pfliiger's Arch. Bd. n. (1869), S. 193. 



3 Maly, Pfliiger's Arch. Bd. ix. (1874), S. 592. 



4 Brucke, Sitzb. d. Wien. Akad. Bd. XLIII. (1861), S. 601. Damlewsky, Virchow's 

 Arch. Bd. xxv. (1862), S. 279. Cohnheim, Virchow's Arch. Bd. xxvni. (1863), S. 241. 



5 Heidenhain, Pfluger's Arch. Bd. x. (1875), S. 583. 



6 Miintz, Compt. Rend. T. LXXX. (1875), p. 1255. 



