ution with its swarming bacteria for
five minutes. In the soft succulent condition in which they exist in
the solution not one of them escapes destruction. The same is true of
the turnip infusion if it be inoculated with the living bacteria
only-the aerial dust being carefully excluded. In both cases the dead
organisms sink to the bottom of the liquid, and without re-inoculation
no fresh organisms will arise. But the case is entirely different when
we inoculate our turnip infusion with the desiccated germinal matter
afloat in the air.

The 'death-point' of bacteria is the maximum temperature at which they
can live, or the minimum temperature at which they cease to live. If,
for example, they survive a temperature of 140 deg., and do not survive a
temperature of 150 deg., the death-point lies somewhere between these two
temperatures. Vaccine lymph, for example, is proved by Messrs.
Braidwood and Vacher to be deprived of its power of infection by brief
exposure to a temperature between 140 deg. and 150 deg. Fahr. This may be
regarded as the death-point of the lymph, or rather of the particles
diffused in the lymph, which constitute the real _contagium_. If no
time, however, be named for the application of the heat, the term
'death-point' is a vague one. An infusion, for example, which will
resist five hours' continuous exposure to the boiling temperature,
will succumb to five days' exposure to a temperature 50 deg. Fahr. below
that of boiling. The fully developed soft bacteria of putrefying
liquids are not only killed by five minutes' boiling, but by less than
a single minute's boiling--indeed, they are slain at about the same
temperature as the vaccine. The same is true of the plastic, active
bacteria of the turnip infusion [Footnote: In my paper in the
'Philosophical Transactions' for 1876, I pointed out and illustrated
experimentally the difference, as regards rapidity of development,
between water-germs and air-germs; the growth from the already
softened water-germs proving to be practically as rapid as from
developed bacteria. This preparedness of the germ for rapid
development is associated with its preparedness for rapid
destruction.]

But, instead of choosing a putrefying liquid for inoculation, let us
prepare and employ our inoculating substance in the following simple
way:-Let a small wisp of hay, desiccated by age, be washed in a glass
of water, and let a perfectly sterilised turnip infusion be inoculated