SCIENCE IN ITS APPLICATION. 93 



access to air, and are called aerobies, in consequence ; 

 others, when cut off from the air, are able to obtain from 

 oxygen-containing compounds all of this element they re- 

 quire for respiration ; such are called anaerobies. Usu- 

 ally, bacteria require a slightly alkaline medium for their 

 development ; only a few can survive in an acid liquid ; 

 whereas, molds require the latter medium for their best 

 growth. When, therefore, the lactic ferment, which 

 sours milk, and the nitrifying ferment, which forms 

 nitric acid in the soil, have produced an excess of acid, 

 they cease to act until the excess is neutralized, when 

 they renew their production of acid. Vinegar, there- 

 fore, serves as a preventive of bacterial fermentation in 

 food preparations. Other substances, conspicuously car- 

 bolic acid, copper and mercury salts, similarly prevent 

 the action of bacteria, and destroy them. 



While diffuse light is not fatal, direct sunshine is 

 the most destructive natural foe of these ferments. 

 They require for their best action certain temperatures, 

 varying for different species. In general, 100 F. is 

 most favorable ; below 50 and above 150 F. few are 

 active, and many are destroyed. The process of pasteur- 

 izing milk by heating to 150 for thirty minutes is based 

 upon this fact. Some bacteria, and especially spores, 

 which are more resistant, owing to their thick walls, are 

 not killed by dry temperatures as low as 315 F., or 

 above 212, the boiling point of water ; very few, how- 

 ever, withstand the latter temperature if they be moist ; 

 consequently, boiling the liquid containing them, or 

 steaming them, are among the most commonly employed 

 methods of sterilization of liquids or solids that is, the 

 destruction of the bacteria the latter contain. 



Bacteria differ, not only in these respects, but in 

 the color, form and consistency of the colonies they 

 make in various liquid and solid media. 



The most sharply distinctive characteristic, how- 



