CHAPTER V. 



BACTERIA IN AIR, SOIL, WATER, MILK. 

 ANTISEPTICS. 



As this work essentially deals with, bacteriology in relation 

 to pathology its scope does not include a full account of the 

 applications of the science to practical sanitation. It is con- 

 venient, however, to give an outline of some of the methods 

 employed in sanitary work and to indicate the chief results 

 obtained. 



Aie. 



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. 



More complete results are available when some method is employed by 

 whi'h the bacteria in a given quantity of air are examined. Thus such 

 a quantity of air may be bubbled by an aspirator through sterile water and 

 measured amounts of this last may be plated on gelatin or other suitable 

 medium. Of the more formal apparatus the following is to be recom- 

 mended : — 



Petri's Sand-Filter Method. — A glass tube open at both ends, and 

 about 3£ inches long and half an inch wide, is taken, and in its centre is 

 placed a transverse diaphragm of very fine iron gauze (Fig. 43, e) ; on each 

 side of this is placed some fine quartz sand which has been burned, well 

 washed, and dried to remove all impurities, and this is kept in position 



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