t may be repeated a
thousand times with the same invariable result. To the naked eye the
liquids at the beginning were alike, being both equally transparent-to
the naked eye they are alike at the end, being both equally muddy.
Instead of putrid mutton-juice, we might take as a source of infection
any one of a hundred other putrid liquids, animal or vegetable. So
long as the liquid contains living bacteria a speck of it communicated
either to the clear mineral solution, or to the clear turnip infusion,
produces in twenty-four hours the effect here described.

We now vary the experiment thus: Opening the back-door of another
closed chamber which has contained for months the pure mineral
solution and the pure turnip infusion side by side, I drop into each
of them a small pinch of laboratory dust. The effect here is tardier
than when the speck of putrid liquid was employed. In three days,
however, after its infection with the dust, the turnip infusion is
muddy, and swarming as before with bacteria. But what about the
mineral solution which, in our first experiment, behaved in a manner
undistinguishable from the turnip-juice? At the end of three days
there is not a bacterium to be found in it. At the end of three weeks
it is equally innocent of bacterial life. We may repeat the experiment
with the solution and the infusion a hundred times with the same
invariable result. Always in the case of the latter the sowing, of the
atmospheric dust yields a crop of bacteria-never in the former does
the dry germinal matter kindle into active life. [Footnote: This is
the deportment of the mineral solution as described by others. My own
experiments would lead me to say that the development of the
bacteria, though exceedingly slow and difficult, is not impossible.]
What is the inference which the reflecting mind must draw from this
experiment? Is it not as clear as day that while both liquids are
able to feed the bacteria and to enable them to increase and multiply,
after they have been once, fully developed, only one of the liquids is
able to develope into active bacteria the germinal dust of the air?

I invite my friend to reflect upon this conclusion he will, I think,
see that there is no escape from it. He may, if he prefers, hold the
opinion, which I consider erroneous, that bacteria exist in the air,
not as germs but as desiccated organisms. The inference remains, that
while the one liquid is able to force the passage from the ina