SPONTANEOUS GENERATION. 577 
ing 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-inquirer 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 ques- 
tions, 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 in- 
voked. Were he now alive, he would, I doubt not, repudi- 
ate the use often made of his name Liebig's view of fer- 
mentation 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 Colin 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 house- 
maid 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 float- 
ing 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 Lon- 
don 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. Mingling with the flame, 
and round its rim, were seen curious wreaths of darkness resembling 
an intensely black smoke. On placing the flame at some distance 
below the beam, the same dark masses stormed upward. They 
were blacker than the blackest smoke ever seen issuing from the 
funnel of a steamer; and their resemblance to smoke was so perfect 
