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Cincinnati Society of Natural History. 



We may observe from the above data that the decrease in 

 the number of bacteria in the air in winter, over fall and 

 spring, is quite plain. Experiments i, 3, 4, 5, 8 and 9 repre- 

 senting autumn conditions, stand in marked contrast to ex- 

 periments 6, 10 and 11 of the winter season. Yet experiments 

 8 and 9 show how great a variation even the winter season 

 may present. It is unfortunate that the conditions of the 

 weather were not recorded in experiment 9, but presumably 

 some disturbing factor entered. Two such factors enter 

 prominently to modify our results. Wind, frequently result- 

 ing in dust-storms in our locality, increases the bacteria in 

 the air, while rain produces the opposite effect. These two 

 factors may neutralize each other wholly or in part and thus 

 modify the final result. The effect of rain is nicely illustrated 

 by the results obtained in experiments 15, 16 and 17. The 

 first exposure was made shortly after a heavy rain, and simi- 

 lar exposures were made on the two following days. The 

 average number of bacteria that fell on the plate in 10 minutes 

 on the three days was 2.5, 14.65 and 40.3. Here we have a 

 constant and material increase as the ground dried off, or as 

 the effect of the rain factor diminished. 



The explanation of the effect produced by rain lies in two 

 directions. First, during the rain the atmosphere is literally 

 washed of impurities, including bacteria. Secondly, the lay- 

 ing of the dust and subsequent soaking of the soil prevents 

 for a period of time the rising of dust and the bacteria con- 

 tained in it. As the dust increases the bacteria again in- 

 crease in the air. 



Ultimately, rain may exert yet another influence. It is 

 well known that bacteria require considerable moisture for 

 reproduction and multiplication. For this reason they do not 

 increase during their journey through the air, but only in 

 moist soil, in decaying bodies, in stagnant pools, etc. It is 

 evident that rain facilitates reproduction by furnishing one of 

 the most essential factors for it, viz., moisture. 



As to the workings of the wind, it is plain that its only 

 effect is to carry dust and its bacteria into the air and to 

 transport them for varying distances. This factor is of great- 

 est importance when dust is most abundant and relatively 

 insignificent after heavy rains. 



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