90 The Story of the Bacteria 



There is another way in which tubercle 

 bacilli are transmitted which though less 

 generally recognized is in some respects more 

 significant than the conveyance by dust or 

 contaminated articles, or by direct contact 

 of the sick with the well, and this is by 

 coughing and sneezing. While the tubercle 

 bacillus can remain alive for some time when 

 dried in dust, it becomes less and less virulent, 

 and strong daylight or direct sunlight kills it 

 in a few days or hours. So that the tubercle 

 bacillus in ordinary dust tends to become less 

 and less a menace. 



In explosive coughing and in sneezing, fine 

 particles in sneezing a veritable spray 

 may be sent forth for several feet into the air. 

 Now in these particles fully virulent tubercle 

 bacilli may be contained, often in great num- 

 bers. The fine spray of sneezing frequently 

 floats for a considerable time in the air and 

 may be breathed in by others. 



Plate X. shows a Petri plate placed three 

 feet from the face during a vigorous sneeze. 

 The colonies growing after four days on the 

 surface of the gelatin show how many living 



