What is 
mildew? 
50 WORLD OF INVISIBLE LIFE 
leather or cotton or woolen materials, they are 
generally called mildew. At first sight there 
does not seem to be much resemblance between 
mildew and mold, but the microscope shows 
that the mildew is nothing more than a certain 
kind of mold that has not grown very vigor¬ 
ously, because the wool or cotton or leather is 
not a good food for it. Other kinds of molds 
grow upon and bring about decay in fruits of 
all kinds. In fact, under the right conditions 
molds will grow on almost every food used by 
man or animals, and on many other things 
besides. 
They will not grow, however, except under 
certain conditions. The greatest necessity of 
their life is moisture. They must have an 
abundance of moisture for a healthy growth, 
and in dry material they will not grow at all. 
This moisture may be in the material on which 
they are growing, or it may be in the air. It 
is well known that molds appear more quickly 
and in larger numbers in rainy and damp 
weather than in dry weather. They will grow, 
