22 THE SMALLEST LIVING THINGS 



bringing about the transformation of nitrous acid to nitric acid 

 and nitrates. 



Man's Control of Bacteria 



Man has learned how to control this mass action of bacteria 

 in many ways and to turn it to good account in his economic and 

 commercial life. Not only has he devised ways to prevent bac- 

 teria from spreading by means of his public hygienic methods and 

 by private prophylaxis, but he has also devised ways to combat 

 the diseases they cause by utilizing the various protective sub- 

 stances that the body develops to overcome infection. Inocula- 

 tion* and vaccination* are to-day used as protective measures in 

 many diseases. By these methods the body is stimulated to pro- 

 duce antitoxins and antibodies to overcome the ravages of the 

 bacteria, and to induce immunity or resistance to disease. 



In industry the activities of many kinds of bacteria have been 

 harnessed. Thus the "curing" of tobacco whereby the special 

 flavoring is obtained, the ripening, with distinctive tastes, of 

 beers and wines, the making of many different kinds of cheese, 

 the flavoring of butter, etc. — all are due in large part to the 

 controlled action of bacteria. Similarly, the retting of flax and 

 the tanning of hides are influenced at many points by bacteria. 



In the disposal of sewage, obviously of the greatest impor- 

 tance for wholesome life in cities, the major part of the treatment 

 of the waste material is performed by bacteria of decay and by 

 the anaerobic bacteria — those bacteria which can live without the 

 presence of free oxygen. For localities without sewerage sys- 

 tems, the modern "septic tank," which utilizes these principles in 

 its construction, has come as a boon for sanitation. 



A great world of practical importance — all dealt with in 

 the science of bacteriology — is thus revealed by the study of the 

 ordinary coccus, bacillus, and spirillum of stagnant water. 

 Thanks to these small creatures other forms of stagnant pool 

 organisms — desmids, diatoms, filamentous algae, etc., among 

 plants, and protozoa, rotifers, Crustacea, worms, etc., among 

 animals — are able to live and thrive, for without the work of the 

 bacteria the continuous interchange of materials between the 

 organic and the inorganic worlds would be impossible. 



* See the Glossary of this book, pages 121 and 124. 



