212 Cincinnati Society of Natural History. 



and in tartar from the teeth. This was in 1675. New life 

 was given to the tottering theory of spontaneous generation 

 by this discovery, and two more centuries of experimentation 

 were required for its final overthrow. In this experimenta- 

 tion the bacteria of the air played an important part, for 

 when life was extinguished in fermenting solutions by heat- 

 ing them, new life soon entered from the air. 



In demonstrating the role played by air bacteria, the classi- 

 cal researches of Pasteur and Tyndall are most interesting. 

 Pasteur had previously shown that nutrient bouillon con- 

 tained in glass vessels, and stoppered with ordinary cotton- 

 wool, would not ferment after sterilization. To show that 

 the ferment which spoiled the medium in the previous experi- 

 ments came from the air, he exposed a large number of tubes 

 containing sterile bouillon, by removing the cotton stoppers 

 for a time and then replacing them again. Nearly all the 

 tubes so exposed underwent fermentation while others kept 

 as controls remained sterile. This experiment was performed 

 near Paris. To show that the ferment (bacteria) varied in 

 quantity in different places, he repeated his experiment in 

 the Alps mountains, and found that only a few of the tubes 

 fermented. 



We have then in Pasteur's work the first evidence that the 

 air of high altitudes contains relatively fewer bacteria than 

 that of lower altitudes. 



Tyndall was primarily interested in the physics of light, 

 and his contributions to biology were merely incidental to 

 physical problems. In order to obtain air free from dust par- 

 ticles, and which should reflect none of the light passing 

 through it, he constructed an air-tight box and covered its 

 inner walls with glycerin. When the box was allowed to 

 stand for some time, the particles of dust settled upon the 

 sides and were held fast by the glycerin. Having become 

 interested in Pasteur's work on air, he was curious to know 

 whether any organic life remained in the air of his box. 

 Sterile nutrient media were exposed in the interior of the box 

 for some time, but they developed no life. Thus he proved 

 in a novel way that air does contain bacteria, and that the 

 putrefaction of the solutions is not due to life arising spon- 

 taneously. 



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