62 FERMENTATION. 



148. Boil keratin with sodium hydrate, filter, and test the 

 filtrate for sulphur by lead acetate. It produces a black precipi- 

 tate of lead sulphid. 



149. Show that keratin responds to the xanthoproteic and 

 Millon's tests. 



FEBMENTATIOK 



By fermentation we mean the decomposition of an 

 organic substance into simpler and more stable molecules, 

 the agent which causes the change being itself unaffected. 

 The agents are living organisms or are formed by such 

 organisms. The living ferments such as the yeast-plant 

 or bacteria are often called the organized ferments. 

 They have the power of reproduction and are composed of 

 cells. The non-living ferments are known as the unor- 

 ganized ferments, or enzymes. They may be excreted by 

 the organized ferments or secreted by living cells, which 

 latter is the case with the digestive ferments. They are 

 not reproductive and act outside of the cell where they 

 were formed. 



The enzymes of the animal cell exist in the cells in an 

 inactive condition, called zymogens, but become active 

 after standing exposed to the atmosphere or being brought 

 in contact with certain chemical compounds. The enzymes 

 contain nitrogen and from their properties are apparently 

 identical with the proteins. They are indiffusible and 

 soluble in glycerin and in water. They can be mechanic- 

 ally removed, without decomposition, from their solutions 

 by forming precipitates therein, to which they adhere, also 

 by saturating the solutions with ammonium sulphate. A 

 low temperature stops their action and they are all killed 

 below 100 if moisture is present. Most enzymes act best 

 at about 38 C. The ferment is not destroyed, but its 



