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continued to be revived from time to time； and experiments were made by
several persons； but these experiments were not altogether satisfactory。
It was found that if you put an infusion in which animalcules would
appear if it were exposed to the air into a vessel and boiled it； and
then sealed up the mouth of the vessel； so that no air； save such as
had been heated to 212 degrees； could reach its contents； that then no
animalcules would be found； but if you took the same vessel and exposed
the infusion to the air； then you would get animalcules。 Furthermore；
it was found that if you connected the mouth of the vessel with a
red…hot tube in such a way that the air would have to pass through the
tube before reaching the infusion； that then you would get no
animalcules。 Yet another thing was noticed： if you took two flasks
containing the same kind of infusion； and left one entirely exposed to
the air； and in the mouth of the other placed a ball of cotton wool； so
that the air would have to filter itself through it before reaching the
infusion； that then； although you might have plenty of animalcules in
the first flask； you would certainly obtain none from the second。
These experiments； you see； all tended towards one conclusionthat the
infusoria were developed from little minute spores or eggs which were
constantly floating in the atmosphere； which lose their power of
germination if subjected to heat。 But one observer now made another
experiment which seemed to go entirely the other way； and puzzled him
altogether。 He took some of this boiled infusion that I have been
speaking of； and by the use of a mercurial batha kind of trough used
in laboratorieshe deftly inverted a vessel containing the infusion
into the mercury； so that the latter reached a little beyond the level
of the mouth of the 'inverted' vessel。 You see that he thus had a
quantity of the infusion shut off from any possible communication 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 youand
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 of
spontaneous generation are altogether due to the falling of minute
germs suspended in the atmosphere；why； I ought not only to be able to
show the germs； but I ought to be able to catch and sow them； and
produce the resulting organisms。〃 He； accordingly； constructed a very
ingenious apparatus to enable him to accomplish this trapping of this
〃germ dust〃 in the air。 He fixed in the window of his room a glass
tube； in the centre of which he had placed a ball of gun…cotton； which；
as you all know； is ordinary cotton…wool； which； from having been
steeped in strong acid； is converted into a substance of great explosive
power。 It is also soluble in alcohol and ether。 One end of the glass
tube was； of course； open to the external air； and at the other end of
it he placed an aspirator； a contrivance for causing a current of the
external air to pass through the tube。 He kept this apparatus going
for four…and…twenty hours； and then removed the 'dusted' gun…cotton；
and dissolved it in alcohol and ether。 He then allowed this to stand
for a few hours； and the result was； that a very fine dust was
gradually deposited at the bottom of it。 That dust； on being
transferred to the stage of a microscope； was found to contain an
enormous number of starch grains。 You know that the materials of our
food and the greater portion of plants are composed of starch； and we
are constantly making use of it in a variety of ways； so that there is
always a quantity of it suspended in the air。 It is these starch
grains which form many of those bright specks that we see dancing in a
ray of light sometimes。 But besides these； M。 Pasteur found also an
immense number of other organic substances such as spores of fungi；
which had been floating about in the air and had got