Pasteur poured meat infusions

Pasteur poured meat infusions into flasks and then drew the
top of each flask into a long curved neck that would admit air but
not dust. He found that if the infusions were heated, they remained
sterile (free from any growth) until they were exposed to dust. He
opened them on a dusty road and resealed them and demonstrated
the growth of microorganisms in all the flasks. The unopened flasks
were sterile.