destroyed the living germs which they contained, the sealed tubes
prevented the air from entering, and when putrefaction or fermentation
of the contents took place the organisms to which this was due, being
contained in the air, entered from without. Objection was made to the
conclusions of Spallanzani that heating the air in the closed tubes so
changed its character as to prevent development of organisms in the
contents. This objection was finally set aside by Pasteur, who showed
that it was not necessary to seal the end of the tube before boiling,
but it could be closed by a plug of cotton wool, which mechanically
removed the organisms from the air which entered the tube, or if the
tube were bent in the shape of a _U_ and the end left open,
organisms from the air could not pass into the tube against gravity
when air movement within the tube was prevented by bending. The
possibility of spontaneous generation cannot be denied, but that it
takes place is against all human experience.

It was not possible to attain any considerable knowledge of the
bacteria discovered by Loewenhoeck until more perfect instruments for
studying them were devised. Lenses for studying objects were used in
remote antiquity, but the compound microscope in which the image made
by the lens is further magnified was not discovered until 1605, and
when first made was so imperfect that the best simple lenses gave
clearer definition. With the betterment of the microscope, increasing
the magnifying power and the sharpness of the image of the object
seen, it became possible to classify the minute organisms according to
size and form and to study the separate species. The microscope has
now reached such a degree of perfection that objects smaller than one
one hundred thousandth of an inch in diameter can be clearly seen and
photographed.