IN THE AIR. 



2 3 



water, or some other substance, to which the particles 

 would adhere ; or merely to hang up such plates in 

 the air for a time, Some observers employed paper 

 moistened with the same fluids. Pasteur investigated 

 the matter very carefully, and showed that the number 

 of germs existing in the air varied much in different 

 places and at different heights. He also found that 

 the air which had been undisturbed for some time in 

 cellars was almost destitute of germs. Mr. Crookes 

 followed a plan which had been previously adopted by 

 Schrceder, and separated germs from the air by filter- 

 ing it through pure cotton wool. He also collected the 

 germs in tubes, and upon glass slides moistened with 

 glycerine (Cattle Plague Report, 1866). It would be 

 generally concluded that many of the germs suspended 

 in air and capable of being wafted long distances by 

 currents, would subside if the air became perfectly 

 still, just as the dust in our rooms falls upon the floor 

 or is deposited upon shelves and other projections. 

 Dr. Tyndall has, however, considered it necessary 

 to demonstrate the fact that tiny particles of dust 

 really do fall down to the ground if the air in which 

 they are suspended be quiescent. By throwing a ray 

 of very bright light upon air contained in a closed 

 chamber of glass 3 ft. by 2 ft. 6 inches, and 5 ft. 

 10 inches high, tapering to a truncated cone at the 

 top, he was able to prove not only that particles of 

 dust which float in the air for a time after it has been 

 disturbed gradually subside, but that these were 



