DUSTS, MOLDS, AND BACTERIA OF THE AIR 85 



92. Bacteria. The bacteria are dependent plants. They 

 are the smallest of living things, often measuring not over 

 "g"o To o" mc ^ m diameter. Some kinds are considerably larger 

 than this, while others are much smaller (fig. 46). Only the 

 very highest powers of the microscope will give information 

 about their internal structures, and there is good reason to 

 believe that there are bacteria so very minute that they are 

 quite invisible even with the best microscope. 



They multiply by simple division. After one has grown to 

 adult size, it may divide into two, each of which may continue 



FIG. 46. Photomicrographs of bacteria 



These photographs were taken through a high-power microscope. The small, dark 

 bodies are the bacteria, very highly magnified. Photographs by Dr. N. M. Harris 



to grow and divide. The division may be repeated as often 

 as once in every twenty minutes, though usually not so fre- 

 quently. Like other dependent plants they feed upon and 

 destroy organic materials, and on account of the rapidity with 

 which they multiply they are usually found in very large 

 numbers. 



93. Bacteriology. The study of bacteria is of great impor- 



tance, and the methods that must be used in studying them 



are different from those used in studying larger plants. This 



study is known as bacteriology. One who wishes to know 



