FERMENTS AND FERMENTATION. 83 



We are only just beginning to get some idea of the chemical 

 composition of a few of the simpler ferments. For this 

 reason the recognition of the existence of a ferment within 

 a cell or out of it is still dependent, not upon the recognition 

 of the chemical substance itself, but rather upon the dis- 

 covery of certain properties common to all ferments and 

 specific ones characteristic of separate enzymes. The fol- 

 lowing are usually looked upon as properties common to all 

 ferments, and the first four are utilized in proving the existence 

 of a ferment in cells, jo reissues, or in extracts madejrfjthese. 



(a) A ferment does not initiate a chemical reaction bid only 

 alters its velocity. The ferment acts essentially through con- 

 tact, in that its mere presence is responsible for .the altered 

 velocity of the catalyzed reaction. At the end of the reaction 

 the ferment is (under ordinary circumstances) found in an 

 unaltered state in the reaction mixture. The ferment does 

 not, therefore, enter into the end-products of the reaction.*' 

 When hydrochloric acid is poured upon sodium carbonate in 

 order to decompose it, we have at the end of the reaction the 

 chlorine of the hydrochloric acid appearing in the sodium 

 chloride, and the hydrogen in the carbonic acid. Not so, how- 

 ever, in the case of a ferment. .When sucrase (invertase, 

 invertin) acts upon cane-sugar, dextrose and laevulose are 

 formed, but neither of these products contains in chemical 

 combination any part or all of the sucrase molecule, the 

 sucrase is found in an unaltered condition at the end of the 

 reaction. 



It is possible that the molecules of the ferment enter tem- 

 porarily into chemical combination with the substance acted 

 upon or with its products. This assumption is made on the 

 ground that in some simple catalytic processes the catalyzer 

 does temporarily combine with the reacting substances. This 

 is the case, for example, in the manufacture of sulphuric acid, 

 in which steam, sulphur dioxide, oxygen, and nitrogen dioxide 

 are introduced simultaneously into a large chamber. In 

 brief, it is believed that the sulphur dioxide and steam com- 



