192 Education through Nature 



One advantage of personal cleanliness is the lessen- 

 ing of the risk of infection and contamination from 

 such injurious microbes. Sterilization, too, by heat 

 or boiling water and disinfectants are for the purpose 

 of destroying disease germs or ferments. The princi- 

 ple of preserving fruits, after boiling and scalding 

 fruit jars, is based on the destruction of these germs 

 by high temperature, and the prevention of the entrance 

 of new ones. Antiseptic surgery, too, is based on 

 this same principle. 



The bacteria contained in the film on the jar of 

 water, after a period of activity, finally come to rest, 

 and secreting a gelatinous substance are held together 

 in solid masses, thus giving rise to the surface film. 

 These forms are harmless. Many of them require 

 oxygen and hence develop most freely near the sur- 

 face. Others do not need oxygen and still others are 

 destroyed by plenty of sunshine and fresh air. They 

 ultimately disappear from the jar when the organic 

 substances are used up. The water will remain pure 

 if the jar be exposed to the sunlight, the activity of 

 the algi keeping the water pure. 



Distinction between Plants and Animals. 



An examination of the jar containing algi, as above, 

 after the water has turned foul, will reveal a white 

 line, of what at first looks like sediment, on the sides 

 of the jar just below the surface of the water, where 

 the latter meets the sides of the vessel. If the jar be 

 placed in the sun, the naked eye can detect millions of 

 rapidly moving bodies, which, when examined with 

 a high power of the microscope, look like little slipper- 

 shaped fishes swimming about very rapidly. These 

 are infusoria, a form called paramecium. They are 

 good examples of microscopic animals called protozoa 

 so called because they correspond to what is called a 



