ion with
the outer air by being inverted upon a bed of mercury.

He then prepared some pure oxygen and nitrogen gases, and passed them
by means of a tube going from the outside of the vessel, up through the
mercury into the infusion; so that he thus had it exposed to a perfectly
pure atmosphere of the same constituents as the external air. Of course,
he expected he would get no infusorial animalcules at all in that
infusion; but, to his great dismay and discomfiture, he found he almost
always did get them.

Furthermore, it has been found that experiments made in the manner
described above answer well with most infusions; but that if you fill
the vessel with boiled milk, and then stop the neck with cotton-wool,
you 'will' have infusoria. So that you see there were two experiments
that brought you to one kind of conclusion, and three to another; which
was a most unsatisfactory state of things to arrive at in a scientific
inquiry.

Some few years after this, the question began to be very hotly discussed
in France. There was M. Pouchet, a professor at Rouen, a very learned
man, but certainly not a very rigid experimentalist. He published a
number of experiments of his own, some of which were very ingenious, to
show that if you went to work in a proper way, there was a truth in
the doctrine of spontaneous generation. Well, it was one of the most
fortunate things in the world that M. Pouchet took up this question,
because it induced a distinguished French chemist, M. Pasteur, to take
up the question on the other side; and he has certainly worked it out
in the most perfect manner. I am glad to say, too, that he has published
his researches in time to enable me to give you an account of them. He
verified all the experiments which I have just mentioned to you--and
then finding those extraordinary anomalies, as in the case of the
mercury bath and the milk, he set himself to work to discover their
nature. In the case of milk he found it to be a question of temperature.
Milk in a fresh state is slightly alkaline; and it is a very curious
circumstance, but this very slight degree of alkalinity seems to have
the effect of preserving the organisms which fall into it from the
air from being destroyed at a temperature of 212 degrees, which is the
boiling point. But if you raise the temperature 10 degrees when you boil
it, the milk behaves like everything else; and if the air with which
it comes in contact, after being boiled at this