Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III HOW THE BACTERIA ARE STUDIED IF you take a small wisp of hay, put it in an open jar, and, covering it with water, set it in a warm place for a day or two, you will presently see that the water which was at first perfectly clear, begins to get turbid, and, after a while, a grayish scum collects on the top. Now the water begins to give off a disagreeable odor of decay. This is what has happened: The bacteria of various forms, which, in the dried condition, were clinging to the hay, or which were in the water, have multiplied to such an extent that they made the water turbid, and many of the mobile forms have sought the surface, where the oxygen was most abundant. The solution of organic material from the hay has furnished an abundance of food, and as the bacteriahave torn this into simpler forms to get the 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 sediment, 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 t...