THE MICROBES IN AIR AND WATER 31 
The fact that light is so fatal to the growth 
of microbes is another reason why air and dust 
are so much less harmful than we should think 
from knowing how many microbes there often 
are floating about in it. It also explains why 
dark places breed disease, and why people, in 
order to keep healthy, need plenty of sunshine. 
Almost all the bodies of water on the earth 
are filled with microbes. Unlike the open air, 
which, as we have seen, is not friendly to the 
growth of microbes, water makes a good home 
for them. It nearly always offers the right 
temperature and abundant food for many kinds 
of microbes. They are washed into it from the 
air and the soil and from the living and dead 
bodies of plants and animals. So almost any 
microbe, harmless or harmful, can sometimes 
be found in it. But, although great numbers 
of microbes flourish in the water, a far larger 
number make their way into it from time to 
time and live only for a certain time. 
In spring-water, which comes fresh from the 
ground, the number of microbes is fewest. 
Can they 
live in water? 
