82 OUR BODIES AND HOW WE LIVE 



when they have served their purpose they fall to pieces of 

 themselves, or from the action of the air, as was once sup- 

 posed ? No ; the microscope has revealed to us whole 

 tribes and families of minute living forms known as yeast, 

 mold, and bacteria. It is the special work of these tiny 

 organisms to change dead animal and vegetable matter 

 into useful substances. In brief, bacteria prepare food 

 for all the rest of the world. 



Were it not for these countless myriads of living organ- 

 isms, the whole surface of the earth would in time be cov- 

 ered with dead trees and other lifeless bodies, and there 

 would be no simple substances left out of which to build 

 up new ones. Were all living forms that are silently at 

 work bringing about these wonderful results destroyed, 

 life upon the earth would be impossible. 



122. Living Forms that cause Disease. Among these 

 minute living forms are some that do not even wait for a 



plant or animal to die 

 before attacking it. 

 Such are the disease 

 germs which are invisi- 



y, .vrv>,- - .-", - ,",' >'- >,\V'-".v ' ;' 



: V^';v V^^vU'^^v ble to the naked eye and 



^ >!._ * .* ' . * . . t ^ \ . > * " : v- -*- - * 



of an infinite variety of 

 form and manner of life. 

 They lurk in the air we 

 breathe ; they occur in 





FIG. 56. Showing the Effect of Variations 



in Temperature on Bacteria Growth. the water we drink; 



a, a single bacterium ; b, its progeny in twenty- they are found in OUr 

 four hours at 50 ; c, its progeny in twenty- j j fc g u j read t Q 



four hours at 70. . r r J J 



cause sickness if they 



gain access to our bodies, unless our bodies are strong 

 enough to resist them. 



