38 
WORLD OF INVISIBLE LIFE 
What is 
yeast? 
would soon be so filled with dead plants and 
animals that living things could not exist. 
Besides the microbes which act on milk to 
help us make butter and cheese, there is the 
very useful group of microbes called yeasts, 
that are used in bread-making and for other 
purposes. Another group of microbes we can 
put to work on the job of making vinegar. 
Other kinds of microbes work for us in the 
tanning of leather, in the “curing” of tobacco, 
and in the preparation of flax for making linens, 
and of hemp for making ropes and cords. 
Probably the most useful of all, however, 
are certain microbes which enrich the soil and 
make it fertile. Every plant and animal 
depends upon the fertility of the soil for its life. 
These microbes live in the soil in great numbers. 
Then, too, there are microbes in the earth 
which actually help to build up iron-ore de¬ 
posits. There are some that perform still other 
useful duties helpful to both man and Nature. 
When we look at milk under a microscope 
we see a surprising thing. We find that it is 
