CHAPTER III 



BACTERIA 



» 



Neither Animals Nor Plants 



AN unsightly scum-covered pool, a green ditch, or a more 

 pleasing pond bordered by lily pads, may be, and usually 

 is, the home of countless millions of the smallest living things. 

 A pint fruit-jar filled from such a source furnishes endless delight 

 to a microscopist, and to him who has patience combined with 

 vision it contains all of the major secrets of life. For the most 

 part these living things are minute, but they vary in size from the 

 limits of vision with the highest powers of the microscope to 

 forms which are just visible to the unaided eye.* Some of the 



A 



o 



B 









Fig. 6— RELATIVE SIZES 



A, a human red blood corpuscle; B, a 

 typhoid bacillus; C, an influenza bacil- 

 lus; D, a common spirillum of stagnant 

 water; E, a giant bacillus from the 

 intestine of a centipede. 



From a drawing by the author 



smallest forms visible with the microscope are bacteria, some of 

 which are tiny spheres known as cocci, some motionless, others 

 moving with microscopic speed through the water. Others, 

 cylindrical in shape, move across the field in a straight or zigzag 



* Measuring these small things is relatively simple, and there are many ways to 

 do it. One simple method is to project, on paper, with the aid of a camera lucida, 

 a scale of one millimeter divided into hundredths. The object to be measured is 

 then similarly projected over the scale and the dimensions read. For greater ac- 

 curacy one of the hundred divisions of the magnified scale may be divided into 

 ten equal parts, each of such parts thus representing one-thousandth of a milli- 

 meter. This is called a micron and is usually accepted as the unit of microscopical 

 size. It is designated by the Greek letter u. (mu), corresponding to our English m. 

 A human red blood corpuscle, which is a convenient object for comparing the sizes 

 of very minute things, measures 6.9 \x in diameter and an influenza bacillus 0.5 u, 

 to 0.2 u.. A millimeter is approximately one twenty-fifth of an inch, and a micron, 

 one twenty-five thousandths of an inch. 



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