78 Germ Theory
Pasteur reasoned that any microbes floating in the air should be concentrated on the
outside of the cotton filter as air was sucked through. Bacterial growth on the filter indicated
microbes floating freely in the air. Bacterial growth in the sterile interior of the tube meant
spontaneous generation.
After 24 hours the outside of his cotton wad turned dingy gray with bacterial growth
while the inside of the tube remained clear. Question number 1 was answered. Yes, microscopic organisms did exist, floating, in the air. Any time they concentrated (as on a cotton
wad) they began to multiply.
Now for question number 2. Pasteur had to prove that microscopic bacteria could not
spontaneously generate.
Pasteur mixed a nutrient-rich bullion (a favorite food of hungry bacteria) in a large
beaker with a long, curving glass neck. He heated the beaker so that the bullion boiled and
the glass glowed. This killed any bacteria already in the bullion or in the air inside the
beaker. Then he quickly stoppered this sterile beaker. Any growth in the beaker now had to
come from spontaneous generation.
He slid the beaker into a small warming oven, used to speed the growth of bacterial
cultures.
Twenty-four hours later, Pasture checked the beaker. All was crystal clear. He checked every
day for eight weeks. Nothing grew at all in the beaker. Bacteria did not spontaneously generate.
Pasteur broke the beaker’s neck and let normal, unsteril