THE CHEMISTRY OF THE CELL 15 



brought about by enzymes or bacteria. Though it will be 

 necessary to refer to certain organisms, the subject will be 

 approached primarily from a chemical standpoint, fermenta- 

 tion being defined as the chemical change produced by the 

 agency of protoplasm or of a secretion prepared from it. 



NOTE TO PAGE 11. The word absorption ' is used advisedly in 

 the sense employed by Dunbar (Principles of Sewage Treatment, p. 142) 

 to signify the power possessed by gelatinous films of withdrawing certain 

 substances from solution, and which is more of the nature of suction 

 than of mere surface attraction, or ' absorption.' 



