wounded lung, should have such wholly different
effects from that entering directly through a wound in the chest, was
to me a complete mystery until I heard of the germ theory of
putrefaction, when it at once occurred to me that it was only natural
that air should be filtered of germs by the air-passages, one of whose
offices is to arrest inhaled particles of dust, and prevent them from
entering the air-cells.

*****

I shall have occasion to refer to this remarkable hypothesis farther
on.

The advocates of the germ theory, both of putrefaction and epidemic
disease, hold that both arise, not from the air, but from something
contained in the air. They hold, moreover, that this 'something' is
not a vapour nor a gas, nor indeed a molecule of any kind, but a
_particle_. [Footnote: As regards size, there is probably no sharp line
of division between molecules and particles; the one gradually shades
into the other. But the distinction that I would draw is this: the
atom or the molecule, if free, is always part of a gas, the particle
is never so. A particle is a bit of liquid or solid matter, formed by
the Aggregation of atoms or molecules.] The term 'particulate 'has
been used in the Reports of the Medical Department of the Privy
Council to describe this supposed constitution of contagious matter;
and Dr. Sanderson's experiments render it in the highest degree
probable, if they do not actually demonstrate, that the virus of
small-pox is 'particulate.' Definite knowledge upon this point is of
exceeding importance, because in the treatment of _particles_ methods
are available which it would be futile to apply to _molecules_.

The Luminous beam as a means of Research.

My own interference with this great question, while sanctioned by
eminent names, has been also an object of varied and ingenious attack.
On this point I will only say that when angry feeling escapes from
behind the intellect, where it may be useful as an urging force, and
places itself athwart the intellect, it is liable to produce all
manner of delusions. Thus my censors, for the most part, have
levelled their remarks against positions which were never assumed, and
against claims which were never made. The simple history of the
matter is this: During the autumn of 1868 I was much occupied with the
observations referred to at the beginning of this discourse, and in
part described in the preceding article. For fifteen years it had
been my habit to mak