92 DUST AND ITS DANGERS. 



garment for weeks or months which could 

 pass unseen through the air and work desola- 

 tion far away was something which might 

 well inspire awe, if not superstition. 



To-day, however, the whole aspect of affairs 

 has changed. We have at last found out that 

 these subtle agencies in the diseases of this 

 class which have been most fully studied, are 

 well-defined organisms which we can isolate 

 and cultivate and study, small as they are, with 

 as much precision and certainty as we can 

 cabbages and pumpkins. We know a great 

 deal about the conditions which favor their 

 growth, and various things which, at least 

 outside of the body, will kill them and render 

 them harmless. 



With this definite knowledge about some of 

 the agents (bacteria) which cause disease, the 

 most impenetrable of the mysteries clustering 

 about the infectious diseases have passed away. 

 For while we do not yet know, as we have seen 

 in another chapter, the exact form or species 

 which is concerned in causing all of the dis- 

 eases of this class, we have indisputable 

 ground for assuming that they all are caused 



