578 Introduction to the Study of Science 



two cells, which carry on the same process indefinitely. Thus a 

 single bacterium may produce many millions within twenty-four 

 hours. The weight of the mass which might be produced from 

 one ancestor in seventy-two hours, if uninterrupted, has been 

 estimated at just a little less than 5000 tons. Is it difficult to 

 understand how fruit, vegetables, and meat pass so quickly from 

 a sound to a decaying or putrefactive condition? Or how a 

 slight wound may, if neglected, quickly become painful and 

 dangerous? The bacterium causing lockjaw, when introduced 

 through a wound in foot or hand, multiplies in the favorable 

 conditions provided in the human body with wonderful rapidity 

 and in a week or ten days may defy curative treatment. 



Habitat. Mention has been made of the extent and abun- 

 dance of bacteria. They are universal and thrive wherever 

 nutritive and other conditions are favorable. They are found 

 in air, water, milk, and meat, and so on throughout the whole 

 list of things which concern human life. Their abundance in 

 the air varies with conditions. Bright sunlight, or the atmos- 

 phere of the desert, high mountains, and mid-ocean are decidedly 

 unfavorable and cause reduction of numbers. Rain washes 

 them from the air ; and prolonged low temperature, as in 

 winter, seems to kill many kinds which do not happen to be 

 protected. Wind scatters them widely. In the country 

 there are comparatively few species and of these a small 

 number; cities show by tests not only greater numbers but 

 more varieties. 



Rooms and workshops occupied by many persons reveal a 

 superabundance of bacteria, which becomes excessive when the 

 air is still or gently moving. Any estimate of the number of 

 bacteria in a given place can be only approximate. In.a cubic 

 meter of outdoor air in a city have been found one hundred 

 fifty bacteria. In an equal volume of air in a room where 

 several people are working, the number varies from fifteen to 

 twenty thousand. In a sleeping room with the usual ventila- 

 tion, occupied by three persons, an equal volume of air con- 



