FERMENT A TION. 54 5 
the spores of the little plant PenicAllium 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 vege- 
table 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 ill 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 to sense as 
dust or powder placed on the palm of the hand. In the 
still air the dust gradually sinks to the floor or sticks to 
the walls and ceiling, until finally, by this self-cleansing 
process, the air is entirely freed from mechanically sus- 
pended matter. 
Thus far, I think, we have made our footing sure. Let 
us proceed. Chop up a beefsteak and allow it to remain 
for two or three hours just covered with warm water; you 
thus extract the juice of the beef in a concentrated form. 
By properly boiling the liquid and filtering it, you can 
obtain from it a perfectly transparent beef-tea. Expose a 
number of vessels containing this tea to the moteless air of 
your chamber; and expose a number of vessels containing 
precisely the same liquid to the dust-laden air. In three 
days every one of the latter stinks, and examined with the 
microscope every one of them is found swarming with the 
bacteria of putrefaction. After three mouths, or three 
