DUST AND ITS DANGERS. 



face of the culture-medium is three square 

 inches in size, it will be readily seen that by 

 the exposure of dishes of the same size for the 



FIG. 2. THE "PLATE METHOD" OF AIR ANALYSIS. 

 The cut shows the appearance of the flat, shallow dish, the bottom 

 of which was covered with nutrient gelatin, and when this had cooled 

 and solidified, was uncovered and exposed to the air in a moderately 

 clean place for five minutes. It was then allowed to stand in a warm 

 place for four days. Immediately after the exposure of the gelatin 

 to the air nothing whatsoever was visible on its surface. But within 

 a few hours tiny spots appeared which grew larger, some more rapidly 

 than others. These " colonies," at the end of four days, when the draw- 

 ing was made, vary considerably in size and appearance, because they 

 are mostly made up of different species of germs. Each colony con- 

 sists of thousands of germs (see Fig. I, A), which have grown on the 

 spot where the lone ancestor fell from the air and stuck fast during 

 the five minutes exposure of the gelatin. 



same time to the air of different places, we 

 can, by comparing the number of bacterial 



