them with food and their potential powers become actual. Such food
is provided by the dead bodies of animals or plants, or by animal
secretions, or from various other sources. The bacteria which are
fortunate enough to get furnished with such food material continue
to feed upon it until the food supply is exhausted or their growth
is checked in some other way. They may be regarded, therefore, as
a constant and universal power usually held in check. With their
universal presence and their powers of producing chemical changes
in food material, they are ever ready to produce changes in the
face of Nature, and to these changes we will now turn.





CHAPTER II.

MISCELLANEOUS USE OF BACTERIA IN THE ARTS.


The foods upon which bacteria live are in endless variety, almost
every product of animal or vegetable life serving to supply their
needs. Some species appear to require somewhat definite kinds of
food, and have therefore rather narrow conditions of life, but the
majority may live upon a great variety of organic compounds. As
they consume the material which serves them as food they produce
chemical changes therein. These changes are largely of a nature
that the chemist knows as decomposition changes. By this is meant
that the bacteria, seizing hold of ingredients which constitute
their food, break them to pieces chemically. The molecule of the