action, when it at once occurred to me that it was
only natural that the 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.'

Here is a surmise which bears the stamp of genius, but which needs
verification. If, for the words 'it is only natural' we were
authorised to write 'it is perfectly certain,' the demonstration would
be complete. Such demonstration is furnished by experiments with a
beam of light. One evening, towards the close of 1869, while pouring
various pure gases across the dusty track of a luminous beam, the
thought occurred to me of using my breath instead of the gases. I
then noticed, for the first time, the extraordinary darkness produced
by the expired air, _towards the end of the expiration_. Permit me to
repeat the experiment in your presence. I fill my lungs with ordinary
air and breathe through a glass tube across the beam. The
condensation of the aqueous vapour of the breath is shown by the
formation of a luminous white cloud of delicate texture. We abolish
this cloud by drying the breath previous to its entering the beam; or,
still more simply, by warming the glass tube. The luminous track of
the beam is for a time uninterrupted by the breath, because the dust
returning from the lungs makes good, in great part, the particles
displaced. After a time, however, an obscure disk appears in the
beam, the darkness of which increases, until finally, towards the end
of the expiration, the beam is, as it were, pierced by an intensely
black hole, in which no particles whatever can be discerned. The
deeper air of the lungs is thus proved to be absolutely free from
suspended matter. It is therefore in the precise condition required
by Professor Lister's explanation. This experiment may be repeated
any number of times with the same result. I think it must be regarded
as a crowning piece of evidence both of the correctness of Professor
Lister's views and of the impotence, as regards vital development, of
optically pure air. [Footnote: Dr. Burden Sanderson draws attention to
the important observation of Brauell, which shows that the _contagium_
of a pregnant animal, suffering from splenic fever, is not found in
the blood of the foetus; the placental apparatus acting as a filter,
and holding back the infective particles.]

Application of Luminous beams to Water.

The method of examination h