CHEMICAL FERMENTS. 583 



starch undergoes a similar transformation to that caused 

 by the action of diastase ; cane-sugar, the same transfor- 

 mation into dextrose and laevulose as by invertin ; super- 

 heated steam splits up fat; continuous boiling with 

 water peptonises albumen. As a difference between the 

 action of these chemical agents and that of ferments, it 

 has been suggested that the latter take such a slight 

 part in the decomposition that they can split up un- 

 limited quantities of the fermentescible bodies ; but as 

 SL matter of fact such an unlimited action does not, by 

 any means, appear to be the case. On the contrary, in 

 the case of pepsin, ptyalin, diastase, &c., there are dis- 

 tinct limits to the action; thus, diastase can, at most, 

 act on two thousand times its quantity of starch. Under 

 -ordinary circumstances it is scarcely likely that all of 

 this power will be employed, because the ferment becomes 

 injured by the alteration of the substratum, or is par- 

 tially precipitated in most fluids by the occurrence of 

 precipitates. 



The sensitiveness of the ferments towards external Dependence 

 influences is comparatively great. As regards the tern- j^ti action 1 " 



perature, in the first place every ferment has an optimum, on external 



. . ,, i i n T influences. 



which, however, is not constant in all cases, but depends 



on other circumstances simultaneously present. In the Temperature. 



case of diastase, the optimum is about 63 C. (Kjeldahl, 



loc. cit.); in the case of emulsin about 50 C. ; in the 



case of ptyalin about 46 C. Diastatic action has been 



observed even at +5 C. ; from that point it increases 



with increasing temperature till it reaches the optimum. 



Between 65 and 75 C. we usually observe a complete 



cessation of the fermentative action, and a permanent 



loss of the fermentative power; the longer the heat is 



continued, so much the lower degrees of temperature 



suffice for this action. The addition of glycerine raises 



the degree of the hurtful temperature; the addition of 



alcohol acts in a converse manner. These numbers, 



however, are only valid as regards moist prepara- 



tions ; in the dry state the isolated ferments can be 



heated from 120 to 160 C. before they are injured. 



Among other circumstances which influence them may 



