If food is allowed to stand for some time, it putrefies .When the putrefied material is examined microscopically ,it is found to be teeming with bacteria. Where do these bacteria come from, since they are not seen in fresh food? Even until the mid-nineteenth century, many people believed that such microorganisms originated by spontaneous (5) generation ,a hypothetical process by which living organisms develop from nonliving matter. The most powerful opponent of the theory of spontaneous generation was the French chemist and microbiologist Louis Pasteur(1822-1895).Pasteur showed that structures present in air closely resemble the microorganisms seen in putrefying materials .He did (10)this by passing air through guncotton filters, the fibers of which stop solid particles. After the guncotton was dissolved in a mixture of alcohol and ether, the particles that it had trapped fell to the bottom of the liquid and were examined on a microscope slide .Pasteur found that in ordinary air these exists a variety of solid structures ranging in size from 0.01 mm to more than 1.0 mm .Many of these bodies resembled the reproductive (15)structures of common molds, single-celled animals, and various other microbial cells. As many as 20 to 30 of them were found in fifteen liters of ordinary air ,and they could not be distinguished from the organisms found in much larger numbers in putrefying materials .Pasteur concluded that the organisms found in putrefying materials originated from the organized bodies present in the air .He postulated that these bodies are constantly (20)being deposited on all objects. Pasteur showed that if a nutrient solution was sealed in a glass flask and heated to boiling to destroy all the living organisms contaminating it, it never putrefied .The proponents of spontaneous generation declared that fresh air was necessary for spontaneous generation and that the air inside the sealed flask was affected in some way (25)by heating so that it would no longer support spontaneous generation. Pasteur constructed a swan-necked flask in which putrefying materials could he heated to boiling, but air could reenter. The bends in the neck prevented microorganisms from getting in the flask.. Material sterilized in such a flask did not putrefy. What does the passage mainly discuss?