f the foundations of a scientific theory of
respiration, and to an examination of the marvellous interactions of
organic substances with oxygen. The presence of free oxygen appeared to
be one of the conditions of the existence of life, and of those singular
changes in organic matters which are known as fermentation and
putrefaction. The question of the generation of the infusory animalcules
thus passed into a new phase. For what might not have happened to the
organic matter of the infusions, or to the oxygen of the air, in
Spallanzani's experiments? What security was there that the development
of life which ought to have taken place had not been checked or prevented
by these changes?

The battle had to be fought again. It was needful to repeat the
experiments under conditions which would make sure that neither the
oxygen of the air, nor the composition of the organic matter, was altered
in such a manner as to interfere with the existence of life.

Schulze and Schwann took up the question from this point of view in 1836
and 1837. The passage of air through red-hot glass tubes, or through
strong sulphuric acid, does not alter the proportion of its oxygen, while
it must needs arrest, or destroy, any organic matter which may be
contained in the air. These experimenters, therefore, contrived
arrangements by which the only air which should come into contact with a
boiled infusion should be such as had either passed through red-hot tubes
or through strong sulphuric acid. The result which they obtained was that
an infusion so treated developed no living things, while, if the same
infusion was afterwards exposed to the air, such things appeared rapidly
and abundantly. The accuracy of these experiments has been alternately
denied and affirmed. Supposing then, to be accepted, however, all that
they really proved was that the treatment to which the air was subjected
destroyed _something_ that was essential to the development of life in
the infusion. This "something" might be gaseous, fluid, or solid; that it
consisted of germs remained only an hypothesis of greater or less
probability.

Contemporaneously with these investigations a remarkable discovery was
made by Cagniard de la Tour. He found that common yeast is composed of a
vast accumulation of minute plants. The fermentation of must, or of wort,
in the fabrication of wine and of beer, is always accompanied by the
rapid growth and multiplication of these _Toruloe_. Thus, ferm