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 investiga- 

 tion 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 contami- 

 nated 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 atmos- 

 pheres in organisms, and if the atmospheres in question be fairly quiescent, 

 then it is sufjficient 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 min- 

 utes the number of bacteria present in lo litres of air are deposited on loo 

 square centimetres of a gelatin plate. 



More complete results are available when some method is employed by 

 which the bacteria in a given quantity of air are examined. The oldest 

 method employed, and one which is still used, is that of Hesse. The 

 apparatus is shown in Fig. 57. It consists of a cylindrical tube a about 

 20 inches long and 2 inches in diameter. At one end this is closed by a 

 rubber cork having a piece of quill tubing / passing through it and projecting 

 some distance into the interior. For use the tube is sterilised in a tall Koch, 



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