THE ACTION OF ENZYMES 69 



Sunlight only affects them to a slight extent. Many 

 poisons, such as chloroform, phenol, thymol, salicylic 

 acid, sodium fluoride, and alcohol, which check the vital 

 activity of organisms or destroy them altogether, have 

 comparatiyely little effect on enzymes. 



By adding one or other of these substances in small 

 amounts to liquid cultures of bacteria it is possible to 

 destroy or check the latter without materially affecting 

 any enzymes they may have formed and set free in the 

 medium. 



3. When fruit juice or a solution containing sugar is 

 exposed to the air in summer it very frequently loses its 

 sweet taste and small bubbles of gas arise within it. 

 The juice becomes frothy, and a chemical examination 

 shows that the sugar is decomposed with the formation 

 of alcohol and carbon dioxide gas, the latter escaping in 

 small bubbles and producing an effect resembling the 

 boiling of water. The peculiar chemical change is found 

 to be due to the presence in the liquid of vast numbers 

 of living yeast-cells, which have grown and multiplied 

 from a few which have fallen in from the air or which 

 were adhering to the fruit when the juice was first pre- 

 pared. The term fermentation (from fervere, to boil) has 

 been applied for a long time to decompositions like this 

 in which the frothing of the liquid is a characteristic 

 phenomenon ; it is, however, now extended to include 

 chemical changes brought about by enzymes such as the 

 diastase, even where no gas is produced, so that its 

 original meaning has disappeared. 



The living yeast plant is sometimes spoken of as an 

 organized ferment in contradiction to the non-living 

 unorganized ferments or enzymes, of which diastase pre- 

 viously mentioned is an example. All early attempts to 



