THE MICROBES IN AIR AND WATER 35 
freezing it, and so ice contains only a few of 
the number that were in the water before it 
was frozen. Most of the harmless microbes die 
within a week. At the end of six months most 
ice is almost entirely free from any kind of 
microbes. And so ice is much purer than the 
water from which it is made. 
We are not sure, however, whether or not 
typhoid fever germs are always killed by freez¬ 
ing even for a long period. Sometimes, they 
seem to have been only stunted in their growth, 
and when the ice is melted, they become active 
again. We know of one instance where ice 
was cut from a river near a place where sewage 
was emptied into it, and when the ice was used, 
seven months later, it caused many cases of 
typhoid fever. Samples of the melted ice 
showed living typhoid germs. But such cases 
are so rare that we can safely say that nearly al¬ 
ways ice is much purer and less dangerous to 
humans than the water from which it is made. 
What we have learned by these studies 
through the microscope has been helpful in the 
Is frozen 
water safe? 
