38 THE FUNDAMENTAL PRINCIPLES. 



ammonia and building up nitrite and nitrate compounds ; these 

 are known as the nitrifying bacteria, and as a class are im- 

 portant, and are being carefully studied. 



IV. Special Chemical Reaction of the Culture -medium. 



Bacteria are likewise profoundly influenced in their growth 

 by the reaction of the medium in which they grow, most bac- 

 teria requiring a neutral or faintly alkaline medium, some few 

 a faintly acid one. This characteristic is found an element 

 of danger for bacterial life : for the bacteria which require 

 alkaline surroundings have, as a rule, the property of secret- 

 ing acids, so that after a while they render the medium unfit 

 for their own further growth long before the pabulum is 

 exhausted. 



THE INERT AND INHIBITIVE CONDITIONS OF BACTERIAL 



LIFE. 



1. Diffuse daylight seems to have little or no influence on 

 bacterial growth, but most bacteria and their spores are killed 

 after a more or less prolonged exposure to the direct rays of 

 the sun, a fact of great importance in practical hygiene. 



2. Electric currents and the X-rays seem to have but little 

 influence on bacterial growth. 



3. Compressed air seems to retard the growth of bacteria. 



4. A number of chemical substances either kill off bacteria 

 or arrest their growth as will be spoken of more fully under 

 the head of Antiseptics and Disinfectants. 



THE VITAL MANIFESTATIONS OR FUNCTIONS OF 

 BACTERIA. 



These are manifold and various, and occasionally attempts 

 at classification have been based on some of these special 

 functions. 



1. Fermentation. The alcoholic and acetic acid fermentation 

 are the work of the yeast fungi ; but the butyric and lactic 

 acid fermentation in milk are caused by a number of bacteria, 



