ion 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 temperature, is passed
through a red-hot tube, you will not get a trace of organisms.

He then turned his attention to the mercury bath, and found on
examination that the surface of the mercury was almost always covered
with a very fine dust. He found that even the mercury itself was
positively full of organic matters; that from being constantly exposed
to the air, it had collected an immense number of these infusorial
organisms from the air. Well, under these circumstances he felt that the
case was quite clear, and that the mercury was not what it had appeared
to M. Schwann to be,--a bar to the admission of these organisms; but
that, in reality, it acted as a reservoir from which the infusion was
immediately supplied with the large quantity that had so puzzled him.

But not content with explaining the experiments of others, M. Pasteur
went to work to satisfy himself completely. He said to himself: "If
my view is right, and if, in point of fact, all these appearances