74 AGRICULTURAL AND INDUSTRIAL BACTERIOLOGY 



isfactorily to destroy the bacteria, present in transparent 

 fluids, such as water, by exposure to the light from some 

 sources having high ultra-violet and violet intensity. This 

 is most effectively accomplished by the use of the Cooper- 

 Hewitt mercury vapor lamp in a quartz tube. Such 

 apparatus has been used on a commercial scale for the 

 purification and sterilization of water. 



Sterilization by Filtration. — Experience has shown that 

 the larger bacteria, at least, are held back when liquids 

 containing them are filtered through porcelain filters 

 (bougies, or candles). These filters in many degrees of 

 fineness (porosity) may be purchased upon the market in 

 many shapes and sizes. Filters are particularly adapted 

 to the sterilization of media which would be decomposed or 

 injured by heat. Many of the materials sterilized by filtra- 

 tion are more or less viscous, such as blood serum, and in 

 consequence do not pass through the filter readily. The 

 rapidity of filtration can be greatly increased by exhausting 

 the air from the flask into which the material is being 

 filtered, the air pressure forcing the material through the 

 pores of the filter. Filters have also been devised whereby 

 it is possible to apply pressure directly to the fluid, forcing 

 it through the pores of the filter with comparative rapidity. 



Culture Media 



The material or substratum upon which an organism is 

 grown is called a medium or substrate. If the organism is 

 to be successfully cultivated it must contain all of the 

 materials essential to its growth. The various species of 

 bacteria, yeasts, molds, and protozoa show wide differences 

 in the nutrient materials which they require. Some forms 

 require inorganic material only; others require a mixture 

 of inorganic and organic substances; and some will grow 

 only in the presence of the most complex proteins or similar 

 materials, under conditions simulating those which exist in 



