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will infect the broth and we will get a growth. Now, in order
that this may be clean, I will pass it through the flame of the
lamp and heat it white hot to be sure that there is nothing
alive on the needle. Then I will take a tube of this broth
and remove its cover. I will say that this rubber cover is
put on simply to prevent evaporation ; it will not protect the
broth from micro-organisms ; they will get in through it or
between it and the glass in some way. It would seem that
it would be hermetically sealed, but it is not ; but this cotton,
the invention of Dr. Schraeder in 1854, forms a filter through
which micro-organisms cannot penetrate ; the air will pass
in and out through the cotton, but the meshes of the cotton
will collect and hold any micro-organisms that come in con-
tact with it. We will burn out the upper end of the cotton
so as to remove any molds that might be on it, because we
may get a shower of spores falling into the tube from any
micro-organism that may be on the surface of the cotton.
Now I will infect that broth and replace the cotton. (Plants
tube No. I.) When I go back to the laboratory I will place
that in the incubator, where it will be kept at the tempera-
ture of 98 degrees until our next lecture, and I will show you
then the changes that have taken place in the broth. It
makes very little difference whose mouth we use to cultivate
from. We get, however, some very peculiar micro-organ-
isms sometimes, and I may say, too, that we often get those
that do not habitually grow in the mouth and will not grow
there for any considerable length of time, for you have all
been gathering them into your saliva this morning coming
to school, for they are in the dust, in the atmosphere and
all around us and about us continually. (Plants tube No. 2.)
If any of you have been eating some bread that has not been
well baked, or drinking some beer that has not been well
filtered, we will be very liable to get some venous yeast ; in
fact, I do not often make a large number of plants from the
mouth without finding some yeast. (Plants tube No. 3.)
Now here is a tube of broth, to which I have added some
litmus. You can perhaps get the color of it all over the room.
It isn't a clear blue, as I said ; it is simply a neutral tint. I
will plant that and we will see what color it will be after a
growth has occurred in it. (Plants tube No. 4.)
Two days ago one of your number came to me with an

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