82 



ELEMENTS OF GENERAL SCIENCE 



to the dust of the air and allowed to remain so for perhaps 

 fifteen minutes, and then covered again ; the other dish should 

 not be uncovered at any time. After several days it will be 

 found that the gelatin hi the dish (fig. 43) which was exposed 

 to the air has upon it a number of small, round spots, which 

 are usually colored white but are sometimes pink, orange, or 

 yellow. These will enlarge from day to day. Study with the 



microscope shows 

 that some of these 

 spots consist of mil- 

 lions of very minute 

 objects which can 

 scarcely be seen even 

 with the aid of the 

 microscope. These 

 minute objects are 

 living plant cells of 

 the simplest struc- 

 ture and are called 

 bacteria. Each of 

 the bacterial colonies 

 has resulted from 

 the multiplication of 

 one or a few bacteria 

 which fell upon the 

 gelatin at that point 

 while it was exposed 

 to the air. 



Therefore we learn from these experiments (1) that a part 

 of the dust of the ah* consists of things that are able to grow 

 and multiply if they happen to alight upon favorable sub- 

 stances; (2) that there are at least three kinds of living 

 things represented in the air bacteria, yeasts, and mold 

 spores; (3) that each of these is able to grow vigorously if 

 only it falls on the right kind of substance ; (4) that all of 



FIG. 43. Colonies of bacteria 



Each of the circular or irregular patches represents 

 one colony of bacteria. The bacteria grew in a thin 

 layer of nutrient agar. The dish is 9 cm. in diame- 

 ter and contains 350 colonies of bacteria. The colo- 

 nies have arisen from the multiplication of bacteria 

 which fell upon the agar during fifteen minutes' 

 exposure out of doors 



