the
silver tube, and in that protected situation will fail to be wiped off
when the instrument is thrust through the tissues. Then when the
trocar is withdrawn, some portions of this dust will naturally remain
upon the margin of the canula, which is left projecting into the
abscess, and nothing is more likely than that some particles may fail
to be washed off by the stream of out-flowing pus, but may be
dislodged when the tube is taken out, and left behind in the cavity.
The germ theory tells us that these particles of dust will be pretty
sure to contain the germs of putrefactive organisms, and if one such
is left in the albuminous liquid, it will rapidly develop at the high
temperature of the body, and account for all the phenomena.

But striking as is the parallel between putrefaction in this instance
and the vinous fermentation, as regards the greatness of the effect
produced, compared with the minuteness and the inertness, chemically
speaking, of the cause, you will naturally desire further evidence of
the similarity of the two processes. You can see with the microscope
the Torula of fermenting must or beer. Is there, you may ask, any
organism to be detected in the putrefying pus? Yes, gentlemen, there
is. If any drop of the putrid matter is examined with a good glass,
it is found to be teeming with myriads of minute jointed bodies,
called vibrios, which indubitably proclaim their vitality by the
energy of their movements. It is not an affair of probability, but a
fact, that the entire mass of that quart of pus has become peopled
with living organisms as the result of the introduction of the canula
and trocar; for the matter first let out was as free from vibrios as
it was from putrefaction. If this be so, the greatness of the
chemical changes that have taken place in the pus ceases to be
surprising. We know that it is one of the chief peculiarities of
living structures that they possess extraordinary powers of effecting
chemical changes in materials in their vicinity, out of all proportion
to their energy as mere chemical compounds. And we can hardly doubt
that the animalcules which have been developed in the albuminous
liquid, and have grown at its expense, must have altered its
constitution, just as we ourselves alter that of the materials on
which we feed. [Footnote: 'Introductory Lecture before the University
of Edinburgh.']

In the operations of Professor Lister care is taken that every portion
of