232 ESSENTIALS OF BACTERIOLOGY 



variety of blights, rots, and new-growths, such as galls attack- 

 ing plants, which are seemingly due to bacteria. About 30 

 varieties have so far been more or less accurately described, 

 but only a few of the organisms have been definitely asso- 

 ciated with the disease. The pear blight is due to Bacillus 

 amylovorus. Crown gall, which affects a great many plants 

 and trees, is supposed to be due to Bacterium tumefaciens; 

 the black rot of cabbage to a pseudomonas. There is much 

 left to be done to place this part of bacteriology on a par 

 with that devoted to animals and man. 



CHAPTER XXXI 

 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 dif- 

 ferences can be attributed to the greater or less quantity of 

 dust and velocity of the wind. 



By the use of balloons, living bacteria have been found at 

 an altitude of 4000 meters. 



Methods of Examination. The simplest method is to 

 expose a Petri dish with gelatin or agar in a dust-laden atmos- 

 phere or in the place to be examined. In the course of twenty- 

 four to forty-eight hours colonies will form 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 purposes somewhat more complicated methods are 

 used, so that a definite amount of air can come in contact 

 with the nutrient medium at a certain regulated rate of speed. 



