76 THE STORY OF THE BACTERIA. 



has been found by actual experiment that a 

 considerable number of living tubercle bacilli 

 may be lodged, together with other dust par- 

 ticles, high up on the walls of hospital wards 

 in which consumptives are unintelligently 

 cared for, in situations to which they could 

 have been conveyed only through the air as 

 ordinary dust is. The same material allowed 

 to dry on handkerchiefs may in a similar way 

 become a source of danger, not only to others, 

 but may cause a fresh infection of the patient 

 himself. 



It is very important to remember that it is 

 only when this discharged material is allowed 

 to dry that it, under ordinary circumstances, 

 becomes a source of infection through the air. 

 Bacteria never rise from thoroughly moist 

 surfaces. One might spread a thick layer of 

 living bacteria of any kind, no matter how in- 

 fectious, over an exposed surface, and, pro- 

 vided it was kept thoroughly moist, might 

 breathe with impunity the air sweeping in 

 strong currents over it, because the germs 

 always cling most tenaciously to such surfaces. 

 Of course a current of air strong enough to 



