20 THE GENERAL CHARACTERS OF MICROORGANISMS 
Relation to Cold—While heat will destroy all bacteria, cold 
will not do so. It is practically impossible to destroy the life of 
bacteria by freezing, at least with any certainty; for no matter 
how low the degree of temperature used, the life of some of these 
organisms seems to be totally resistant. Experiments have shown 
that bacteria cooled to the temperature of liquid air, or even liquid 
hydrogen, are not all killed, but after being warmed are still able 
to germinate. Although these extremes of temperature do not 
destroy all bacteria, the simple matter of freezing and thawing will 
kill a great number of them. If water containing a large number 
of bacteria is frozen and subsequently thawed, the bacteria will 
be found much reduced in numbers, although they are not by 
any means all killed. When, therefore, water is contaminated by 
sewage containing typhoid bacteria, and ice is collected from it 
for domestic purposes, the typhoid bacteria may still be found 
alive in the ice. Such ice may still be a source of danger. But it 
must also be remembered that freezing destroys a very large 
proportion of these germs, so that the danger from the ice is far 
less than from the water before it was frozen. 
Relation to Air.—Nearly all living organisms require air, and 
it was formerly supposed that nothing could live without it. 
Certain types of bacteria, however, are able to live without 
air. Indeed, some species, while they grow readily if they have 
no contact with air, fail to grow at all when the slightest amount 
of air is present, growing only in the absence of oxygen. This 
type of bacteria is spoken of as strictly anaerobic. At the other 
extreme, there is a long list of bacteria which can grow only in 
the presence of air, failing to grow if they do not have oxygen at 
their command. This is the type of strictly aerobic bacteria. 
Between the two is an intermediate group capable of growing 
either in the air or out of contact with it, and these are spoken of 
as facultative anaerobic. 
Relation to Moistwre——Bacteria will grow only in the presence 
of considerable quantities of moisture; indeed, they demand 
