142 PA THOGENIC BA CTERIA. 



of bacteria is considerably increased : if the person be a 

 woman with skirts, more bacteria will probably be raised 

 from the floor than would be disturbed by a man ; if the 

 room be swept, the increase is enormous. From these 

 and similar contingencies it becomes very difficult to 

 know just when and how the air is to be examined, 

 and the value of the results is correspondingly lessened. 

 The most valuable examinations are those which aim 

 at the discovery of some definite organism or organisms 

 regardless of the number per cubic meter. 



