h centuries,
hypotheses did not stimulate research, but led to further
speculations. As late as 1820 Ozanam expressed himself as follows:
"Many authors have written concerning the animal nature of the
contagion of disease; many have assumed it to be developed from animal
substance, and that it is itself animal and possesses the property of
life. I shall not waste time in refuting these absurd hypotheses." The
theory of a living contagion was too simple, and not sufficiently
related to the problems of the universe to serve the medical
philosophers.

Knowledge of the minute organisms was slowly accumulating. The first
questions to be determined were as to their nature and origin. How
were they produced? Did they come from bodies of the same sort
according to the general laws governing the production of living
things, or did they arise spontaneously? a question which could not be
solved by speculation but by experiment. The first experiments, by
Needham, 1745, pointed to the spontaneous origin of the organisms. He
enclosed various substances in carefully sealed watch crystals from
which the air was excluded, and found that animalculi appeared in the
substance, and argued from this that they developed spontaneously. In
1769, Spallanzani, a skilled experimental physiologist, in a brilliant
series of experiments showed the imperfect character of Needham's work
and the fallacy of his conclusions. Spallanzani placed fluids, which
easily became putrid, in glass tubes, which he then hermetically
sealed and boiled. He found that the fluid remained clear and
unchanged; if, however, he broke the sealed point of such a tube and
allowed the air to enter, putrefaction, or in some cases fermentation,
of the contents took place. He concluded that boiling the substances
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 tub