614 



BACTERIA IX THE AIR. 



mined by investigation more bacteria are found near the surface of 

 the earth than at some distance above the surface, more over the 

 land than over the ocean, more in cities with their dust-covered 

 streets than in the country with its grass-covered fields. 



Careful experiments have shown that bacteria do rot find their 

 way into the atmosphere from the surface of liquids, unless portions 

 of the liquid containing them are projected into the air by some 

 mechanical means, such as the bursting of bubbles of gas. Cultures 

 of pathogenic bacteria freely exposed to the air in laboratories do not 

 endanger the health of those who work over them ; but if such a cul- 

 ture is spilled upon the floor and allowed to remain without disin- 

 fection, when it is desiccated the bacteria 

 contained in it will form part of the dust of 

 the room and might be dangerous to its 

 occupants. Bacteria do not escape into the 

 air from the surface of the fluid contents of 

 sewers and cesspools, but changes of level 

 may cause a deposit upon surfaces, which 

 is rich in bacteria, and when dried this ma- 

 terial is easily carried into the atmosphere 

 by currents of air. 



Tyndall's experiments (1869) show that 

 in a closed receptacle in which the air is 

 perfectly still all suspended particles are af- 

 ter a time deposited on the floor of the closed 

 air chamber. And common experience de- 

 monstrates the fact that the dust of the at- 

 mosphere is carried by the wind from ex- 

 posed surfaces and again deposited when the 

 air is at rest. This dust as deposited, for 

 example, in our dwellings contains innu- 

 merable bacteria in a desiccated condition, 

 and the smallest quantity of it introduced 

 into a sterile organic liquid will cause it to 

 undergo putrefactive decomposition, and 

 by bacteriological methods it will be found 

 to contain various species of bacteria. Such 

 dust also contains the spores of various 

 mould fungi which are present in the atmo- 

 sphere, usually in greater numbers than the 

 bacteria. The mould fungi are air plants 



which vegetate upon the surface of moist organic material and form 

 innumerable spores, which are easily wafted into the air, both on 

 account of their low specific gravity and minute size, and because they 



FIG. 186. Penicillium glau- 

 cum; m, mycelium, from which 

 is given off a branching pedicle 

 bearing spores. X 150. 



