How we may isolate Bacteria of Certain Kinds from Others.--In order to get a number of bacteria of a given kind to study, it becomes necessary to grow them in what is known as a pure culture. This is done by first growing the bacteria in some medium such as beef broth, gelatin, or on potato.[20] Then as growth follows the colonies of bacteria appear in the culture media or the beef broth becomes cloudy. If now we wish to study one given form, it becomes necessary to isolate them from the others. This is done by the following process: a platinum needle is first passed through a flame to _sterilize_ it; that is, to kill all living things that may be on the needle point. Then the needle, which cools very quickly, is dipped in a colony containing the bacteria we wish to study. This mass of bacteria is quickly transferred to another sterilized plate, and this plate is immediately covered to prevent any other forms of bacteria from entering. When we have succeeded in isolating a certain kind of bacterium in a given dish, we are said to have a _pure culture_. Having obtained a pure culture of bacteria, they may easily be studied under the compound microscope. Footnote 20: For directions for making a culture medium, see Hunter, _Laboratory Problems in Civic Biology_. Culture tubes may be obtained, already prepared, from Parke, Davis, and Company or other good chemists. [Illustration: A pure culture of bacteria. Notice that the bacteria are all the same size and shape.] Size and Form.--In size, bacteria are the most minute plants known. A bacterium of average size is about 1/10000 of an inch in length, and perhaps 1/50000 of an inch in diameter. Some species are much larger, others smaller. A common spherical form is 1/50000 of an inch in diameter. They are so small that several million are often found in a single drop of impure water or sour milk. Three well-defined forms of bacteria are recognized: a spherical form called a _coccus_, a rod-shaped bacterium, the _bacillus_, and a spiral form, the _spirillum_. Some bacteria are capable of movement when living in a fluid. Such movement is caused by tiny lashlike threads of protoplasm called _flagella_. The flagella project from the body, and by a rapid movement cause locomotion to take place. Bacteria reproduce with almost incredible rapidity. It is estimated that a single bacterium, by a process of division called _fission_, will give rise to over 16,700,000 others in twenty-four hours. Under unfavorable conditions they stop dividing and form rounded bodies called spores. This spore is usually protected by a wall and may withstand very unfavorable conditions of dryness or heat; even boiling for several minutes will not kill some forms.