1 6 DUST AND ITS DANGERS. 



medium and allowing it to solidify by cooling. 

 This gives a smooth, moist, somewhat adhesive 

 surface of equal size in each of the dishes, 

 which are immediately protected from any 

 chance contamination by closely-fitting glass 

 covers. 



This mode of air analysis depends upon the 

 fact which we have mentioned above, and 

 which everybody is familiar with, namely, that 

 all dust particles, light or heavy, in quiet places, 

 slowly but surely settle towards the ground. 

 If now we set one of our covered dishes in a 

 still place and take off the cover, the dust 

 particles, the inorganic as well as the living, 

 will settle on to this moist nutrient surface. 

 With the inorganic components of the dust, the 

 multifarious shreds and patches of one thing 

 or another, this is the end of the matter. But 

 as the living dust particles touch the surface, 

 like Antaeus, they find their abeyant vigor 

 quickly renewed, and forthwith commence to 

 multiply and inherit their little new-found 

 earth. Now, suppose we leave our dishes un- 

 covered and exposed to the falling dust for, 

 say five minutes ; suppose further that the sur- 



