102 THE FERMENTS. 



activity of the ferments is further decreased with an increase of their 

 specific products beyond a certain degree. Absence of water like- 

 wise inhibits the action of the ferments, but this is at once re- 

 established when the necessary degree of moisture is supplied, and 

 it is possible therefore to preserve the ferments in the dry state. 

 During the process of drying, however, care must be had that the 

 temperature does not exceed a certain limit. This varies with the 

 different ferments, but it may be stated as a general rule that all 

 animal ferments are killed by a temperature of 75 C., while the 

 vegetable ferments cannot survive a temperature of 80 C. In the 

 absence of moisture, however, they can apparently withstand much 

 greater heat, and it is said that dry trypsin, pepsin, and diastase may 

 be heated to a temperature of from 150 to 160 C. without losing 

 their activity. Strong alcohol destroys the action of certain ferments, 

 such as pepsin and diastase, while others, like the fibrin-ferment, are 

 not affected. 



The most peculiar property of the ferments, and the one which is 

 characteristic of all, is the power to bring about an amount of 

 chemical change which is out of all proportion to the quantity of the 

 ferment present, while the ferment itself undergoes no apparent 

 change. The common pepsin preparations of the market are thus 

 of a strength that 1 part by weight of the pepsin will digest 6000 

 parts by weight of coagulated egg-albumin, and Petit claims that a 

 preparation from his laboratory was capable of dissolving even 

 500,000 times its weight of fibrin in seven hours. 



That the ferments themselves undergo no change while exerting 

 their specific action can be readily shown, as it is possible to re- 

 obtain them from the various digestive mixtures and to test their 

 efficacy as before. 



The rapidity with which the action of ferments takes place is 

 often most remarkable, and is especially well shown during the 

 coagulation of milk under the influence of chymosin. 



In order that the ferments may exhibit their activity to best 

 advantage a definite temperature is necessary, which varies some- 

 what with the different ferments, but is generally about that of the 

 body. Higher as well as lower temperatures gradually inhibit their 

 action, and, as has been seen, destroy it entirely w r hen 75 80 C. is 

 reached. Very low temperature has the same effect. 



The presence or absence of oxygen has no effect upon the action 

 of ferments, and they thus show a distinct difference from the 

 organized ferments, which are more or less dependent upon either 

 its presence or its absence. 



The reaction of the medium in which the ferments are to display 

 their activity is very important, and varies with the different fer- 

 ments. Some of these, such as pepsin, can act to advantage only 

 in an acid medium ; while others, such as ptyalin, require an alka- 

 line reaction ; and still others can act in acid, alkaline, and neutral 

 media, but exhibit certain preferences. 



