s as the
germs of the life observed?

The name of Baron Liebig has been constantly mixed up with these
discussions. 'We have,' it is said, 'his authority for assuming that
dead decaying matter can produce fermentation.' True, but with Liebig
fermentation was by no means synonymous with life. It meant,
according to him, the shaking asunder by chemical disturbance of
unstable molecules. Does the life of our flasks, then, proceed from
dead particles? If my co-enquirer should reply 'Yes,' then I would
ask him, 'What warrant does Nature offer for such an assumption?
Where, amid the multitude of vital phenomena in which her operations
have been clearly traced, is the slightest countenance given to the
notion that the sowing of dead particles can produce a living crop?'
With regard to Baron Liebig, had he studied the revelations of the
microscope in relation to these questions, a mind so penetrating could
never have missed the significance of the facts revealed. He,
however, neglected the microscope, and fell into error--but not into
error so gross as that in support of which his authority has been
invoked. Were he now alive, he would, I doubt not, repudiate the use
often made of his name--Liebig's view of fermentation was at least a
scientific one, founded on profound conceptions of molecular
instability. But this view by no means involves the notion that the
planting of dead particles--'Stickstoffsplittern' as Cohn
contemptuously calls them--is followed by the sprouting of infusorial
life.

*****

Let us now return to London and fix our attention on the dust of its
air. Suppose a room in which the housemaid has just finished her work
to be completely closed, with the exception of an aperture in a
shutter through which a sunbeam enters and crosses the room. The
floating dust reveals the track of the light. Let a lens be placed in
the aperture to condense the beam. Its parallel rays are now
converged to a cone, at the apex of which the dust is raised to almost
unbroken whiteness by the intensity of its illumination. Defended
from all glare, the eye is peculiarly sensitive to this, scattered
light. The floating dust of London rooms is organic, and may be
burned without leaving visible residue. The action of a spirit-lamp
flame upon the floating matter has been elsewhere thus described:

*****

In a cylindrical beam which strongly illuminated the dust of our
laboratory, I placed an ignited spirit-lamp. Min