2l6 ESSENTIALS OF BACTERIOLOGY 



CHAPTER XXIII 

 EXAMINATION OF AIR, SOIL, AND WATER 



Air. Many germs are constantly found in the atmosphere 

 about us. Bacteria unaided do not rise into the air and fly 

 about; they usually become mixed with small particles of dirt 

 or dust and are moved with the wind. The more dust, the 

 more bacteria, and, therefore, the air in summer contains a 

 greater number than the air in winter, and all the other differ- 

 ences can be attributed to the greater or less quantity of dust 

 and wind. 



Methods of Examination. The simplest method is to 

 expose a glass or dish covered with gelatin in a dust-laden 

 atmosphere or in the place to be examined. In the course of 

 twenty-four to forty-eight hours colonies will be seen formed 

 wherever a germ has fallen. But this method will not give any 

 accurate results in regard to the number of bacteria in a given 

 space; for such a purpose somewhat more complicated methods 

 are needed, so that a certain amount of air can come in contact 

 with the culture-media at a certain regulated rate of speed. 



Hesse's Method. This is the oldest and most useful of 

 the various methods in vogue. 



A glass cylinder, 70 cm. long and 3.5 cm. in diameter, is 

 covered at one end by two rubber caps, the inner one having 

 a hole in its center 10 mm. in diameter; and at the end B, a 

 rubber cork fits in the cylinder; through this cork a glass tube 

 10 mm. in diameter passes, which is plugged at both ends with 

 cotton. The cylinder and fittings are first washed in alcohol 

 and sublimate and then placed for one hour in the steam- 

 chamber. 



Removing the cork of the cylinder, 50 c.c. of sterile gelatin 

 in a fluid condition are introduced and rolled out on the sides 

 of the tube, after the manner of Esmarch, leaving a somewhat 

 thicker coating along the under side of the cylinder. The 

 aeroscope, as the cylinder and its fittings are called, is placed 



