26 THE STORY OF THE BACTERIA. 



particular elements which they needed for 

 their own use, the freed material, in part in 

 the form of bad-smelling gases, has either been 

 set free into the air or remains absorbed in the 

 water. 



If you examine a tiny droplet of the water 

 from time to time with the microscope, you 

 will find that it is swarming with various forms 

 of bacteria, rods, balls, and perhaps spirals, 

 many of them in active motion. But you will 

 notice that from day to day the prevailing 

 forms change. One day the little rods will be 

 most abundant, the next these may have 

 largely disappeared, and perhaps the little 

 balls are the most common forms. Then 

 perhaps a new set of rods or balls will appear 

 of a different size from the first. After a 

 while you will find that the bottom of the jar 

 has become covered with a light-colored sedi- 

 ment, and the water has become clearer. 



The bacteria of one form or another have 

 gone on dividing and subdividing, breaking up 

 the dissolved organic matter in the water until 

 either they had used up the special form of 

 material which was best suited to their needs, 



