they dwindle to
the size of impalpable dust particles; assuming that you treat them
all in the same way, and that from every one of them in a few days you
obtain a definite crop--may be clover, it may be mustard, it may be
mignonette, it may be a plant more minute than any of these, smallness
of the particles, or of the plants that spring from them, does not
affect the validity of the conclusion. Without a shadow of misgiving
you would conclude that the powder must have contained the seeds or
germs of the life observed. There is not in the range of physical
science, an experiment more conclusive nor an inference safer than
this one.

Supposing the powder to be light enough to float in the air, and that
you are enabled to see it there just as plainly as you saw the heavier
powder in the palm of hand. If the dust sown by the air instead of by
the hand produce a definite living crop, with the same logical rigour
you would conclude that the germs of this crop must be mixed with the
dust. To take an illustration: the spores of the little plant
Penicillium glaucum, to which I have already referred, are light
enough to float in the air. A cut apple, a pear, a tomato, a slice of
vegetable marrow, or, as already mentioned, an old moist boot, a dish
of paste, or a pot of jam, constitutes a proper soil for the
Penicillium. Now, if it could be proved that the dust of the air when
sown in this soil produces this plant, while, wanting the dust,
neither the air, nor the soil, nor both together can produce it, it
would be obviously just as certain in this case that the floating dust
contains the germs of Penicillium as that the powders sown in your
garden contained the germs of the plants which sprung from them.

But how is the floating dust to be rendered visible? In this way.
Build a little chamber and provide it with a door, windows, and
window-shutters. Let an aperture be made in one of the shutters
through which a sunbeam can pass. Close the door and windows so that
no light shall enter save through the hole in the shutter. The track
of the sunbeam is at first perfectly plain and vivid in the air of the
room. If all disturbance of the air of the chamber be avoided, the
luminous track will become fainter and fainter, until at last it
disappears absolutely, and no trace of the beam is to be seen. What
rendered the beam visible at first? The floating dust of the air,
which, thus illuminated and observed, is as palpable