1 8 DUST AND ITS DANGERS. 



colonies which develop on the surfaces, get at 

 least an approximate idea of the relative num- 

 ber of suspended bacteria slowly settling in the 

 air of the different places (see Fig. 2). 



We cannot, of course, by this method say 

 how many germs were present in a given vol- 

 ume of air, as we can by the more elaborate 

 and accurate method given above, and there 

 are many minor sources of error. For exam- 

 ple, the mould spores are so very light and 

 buoyant that they fall but slowly, so that we 

 may altogether miss many of them, and the 

 same may be true of some of the lighter bac- 

 teria. Moreover, even very slight upward air 

 currents may interfere with the settling of the 

 germs, and in windy places this method is of 

 little use. But on the whole, if similar condi- 

 tions are maintained in the different analyses, 

 comparative results may be obtained in this 

 way which are of much value, as we shall pres- 

 ently see. 



This, which we will call the "plate-method," 

 enables us to get a general notion of the bac- 

 terial contents of the air in various places under 

 conditions which would render the use of the 



