396 ELEMENTARY BIOLOGY 



and drainage are subject to regulation, as well as the disposal of 

 garbage, household and industrial refuse, ashes, etc. In many 

 towns the scattering of ashes or other dust and the pollution 

 of the air with smoke are treated as public nuisances. 



The prohibition of spitting in public places has come to be 

 a matter of course in all wide-awake communities, and the same 

 is true of the use of public towels. The public is also coming 

 to insist that street cars, boats, and other public conveyances be 

 kept thoroughly clean and sanitary. In many towns the public 

 is making provision for baths that are either entirely free for 

 all to use or open for a nominal fee. 



435. Uses of bacteria. We have studied the changes thai 

 bacteria produce in dead organic matter, making the elements 

 of the latter again available for the living plants, and so for 

 animals. We have also noted the importance of certain bac- 

 teria in making the atmospheric nitrogen available for our 

 growing crops. 



There is still another way in which bacteria make dead 

 organic matter in the soil and in waters available as food. The 

 bacteria themselves, feeding upon the dead remains, are in 

 turn eaten by various protozoa and other minute animals. 

 These are then eaten by larger animals, and so on until we 

 get to forms that are large enough to serve as food for man, 

 as shrimps, clams, fish, etc. 



436. Bacteria in industry. The decay caused by various bac- 

 teria is utilized directly in the preparation of sponges for 

 commerce. The sponges are allowed to lie in tanks of water 

 until the dead cells are completely destroyed by bacteria. They 

 are then washed clean, leaving the horny skeletons with which 

 we are familiar. A similar process, involving the activity of 

 different kinds of bacteria, is employed in the '" retting " (really 

 " rotting ") of the soft portions of flax and hemp stalks, to 

 facilitate the separation of the fibers. 



The action of bacteria enzymes is used in the making of 

 vmegar out of cider, wine, or other liquids containing alcohol. 



