26 
WORLD OF INVISIBLE LIFE 
What can 
destroy them? 
is frozen, they nearly always die in a few weeks, 
even though the temperature is not far below 
freezing. 
The majority of the ordinary microbes are 
quickly killed by dry air, though there are great 
differences among the various forms. Exposure 
to dryness for a few hours, or at most, a few 
days, destroys most of the harmful microbes, 
so that the catching of disease-germs through 
the air, except when the microbes are enclosed 
in drops of moisture, is rather rare. The tuber¬ 
culosis germ is one of those which can resist 
dryness the longest, and the cholera germ is 
one of those which die most quickly in a dry 
place. Some of the more complex forms of 
invisible life, like certain water-worms, known 
as rotifers, can live in a dry place for months 
or even years, for they are protected by a gelatin 
covering, which makes it possible for them to 
exist under dry conditions. The microbe does 
not have this protective covering, thus its re¬ 
moval from moisture speedily destroys its life. 
We all know that most living things need 
