614 BACTERIA IN 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 not 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 allsuspended 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 
tocontain various species of bacteria. Such 
Fic. 186.—Penicilium. glau- dust also contains the spores of various 
cum; m, mycelium, from which ould fungi which are present in the atmo- 
is given off a branching pedicle “ 
bearing spores. x 150. 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 
