CHAPTER IV. 



BACTERIA IN AIR, SOIL, AND WATER. 

 ANTISEPTICS. 



IT is impossible here to do more than indicate the chief methods 

 which are employed by bacteriologists in the investigation of the 

 bacteria present in air, soil, and water, and to add an outline of 

 the chief results obtained. In dealing with the latter the subject 

 has been approached mainly from the standpoint of the bearings 

 which the results have towards human pathology. In dealing 

 with antiseptics, so far as possible the effects of the various 

 agents on the chief pathogenic bacteria have been given, though 

 in many cases our information is very imperfect. 



AIR. 



Very little information of value can be obtained from the 

 examination of the air, but the following are the chief methods 

 used, along with the results obtained. More can be learned 

 from the examination of atmospheres experimentally contamin- 

 ated than by the investigation of the air as it exists under 

 natural conditions. 



Methods of Examination. The methods employed vary with the 

 objects in view. If it be sought to compare the relative richness of 

 different atmospheres in organisms, and if the atmospheres in question 

 be fairly quiescent, then it is sufficient to expose gelatin plates for 

 definite times in the rooms to be examined. Bacteria, or the particles of 

 dust carrying them, fall on the plates, and from the number of colonies 

 which develop a rough idea of the richness of the air in bacteria can be 

 obtained. Petri states that in five minutes the bacteria present in 

 10 litres of air are deposited on 100 square centimetres of a gelatin 

 plate. 



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