BACTERIA. 53 
We have next to consider the meaning of the scum which as 
we saw forms on the surface of the decomposing fluid at a certain 
stage. The examination of a bit of this shows that it consists of 
Bacteria which have come to rest, lost their cilia, and lie imbedded 
in a gelatinous substance which they have probably thrown out 
around themselves in something the same way as a slug throws 
out its slime. : 
Some of these quiescent Bacteria—the process is best seen 
in Bacilli—become greatly elongated, forming long intertwined 
threads. The protoplasm in these collects itself into little 
rounded masses, which surround themselves each with a thick 
coat of cellulose. Finally the filament breaks up, and the 
rounded masses or spores are left. When the scum falls to the 
bottom, the spores remain in it unaltered, being protected by 
their thick coat, and may, remain alive, though inactive, for an 
indefinite period. These spores are much more difficult to kill 
than the adult Bacteria ; treatment which invariably destroys the 
latter has no effect on the former. If the fluid is allowed to dry 
up—as such fluids are constantly doing—the dried scum is left 
as a papery mass, which is blown into dust at every puff of wind. 
If these spores are placed in a proper nutrient fluid, they send 
out a sort of bud, and are soon transformed into ordinary resting 
Bacteria. 
Now comes the question—how do the Bacteria find their way 
into these fluids? It may be said either that they or their spores 
exist in the hay, or that the spores existed in the air as fine 
dust and were carried by currents into the fluid, or that they 
were created anew in the fluid. This last is the theory of Spon- 
taneous Generation. 
The first of these answers is undoubtedly true, for very careful 
microscopical examination has revealed the presence of spores on 
hay, but that it will not account for the whole of the facts is 
proved by boiling hay-infusion for a considerable time—so as to 
kill any Bacteria which may be contained in it—and then placing 
it aside either completely exposed to the air or lightly covered. 
Sooner or later it will be turbid and swarming with Bacteria. 
And what is true of hay-water is equally true of all other organic 
infusions: boil them for any length of time and set them aside 
either completely exposed or in ordinary corked or even stop- 
pered bottles, and before long they will be found to be a mass of 
corruption, teeming with Bacteria. 
Thus in fluids in which the Bacteria have been destroyed by 
boiling it may be taken for granted that subsequent decompo- 
sition is due (@) either to Bacteria being carried to them from the 
air, or (4) to their having been spontaneously generated therein. 
That germs of various sorts do exist in the air has been proved 
by collecting floating dust from the air. This, examined micro- 
scopically, is found to contain spores of various kinds, and when it 
is added with proper precautions to a boiled organic infusion, 
putrefaction always ensues. 
