90 



ELEMENTS OF GENERAL SCIENCE 



another interesting experiment may be performed. The mate- 

 rial must have been heated to destroy any living things that 

 may have been in it. One of these dishes should be uncov- 

 ered 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 in 

 the dish which was 

 exposed to the air 

 has upon it a num- 

 ber of small round 

 spots (fig. 45) which 

 are usually colored 

 white but are some- 

 times pink, orange, 

 or yellow. These 

 spots will enlarge 

 from day to day. 

 They consist of mil- 

 lions of very mi- 

 nute objects which 

 can scarcely be seen 

 even with the aid 

 of the microscope. 

 These are living 

 plant cells of the 

 simplest structure 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 when it 

 was exposed to the air. 



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

 of the dust of the air 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 



FIG. 45. 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 



