304 ENZYMES. [CH. XVI 



material has been killed and is completely disintegrated, 

 so that cases may occur where material suspended in 

 water will show ferment action, when an extract will not. 



Dilute glycerin may be used instead of water for 

 extracting ferments ; if this is used the solution is pre- 

 cipitated by alcohol and the precipitate dissolved in water. 



The amount of ferment present in a solution cannot 

 be determined, but the amount of a particular change 

 produced by different solutions containing a ferment may 

 be compared. 



Qualitative examination. 



The presence of ferments is shown by allowing the 

 extract to stand in contact with the substances on which it 

 is supposed to be capable of acting for some hours at 

 30° — 50° C, and then testing for characteristic products. 



A parallel experiment is made at the same time with 

 another portion of the extract which has been boiled 

 before using, to destroy the ferment ; if none of the 

 characteristic product is produced in this case, the action 

 of the original extract may be attributed to ferment. 



Quantitative. 



Equal quantities of two extracts are allowed to act, 

 under similar conditions, for equal times on equal weights 

 of the substances to be acted on, and one of the products 

 of the change is estimated. 



The activity of the two solutions will be roughly 

 proportional to the amounts of the product formed. 



Kjeldahl has shown that methods based on measuring 



