BACTERIA. 103 
sands of infusions of all sorts exposed to exactly the same con- 
ditions as this infusion of old hay, have shown no development 
of life: is it not more likely that, owing to some at present un- 
known cause, the germs in this infusion were not killed by boil- 
ing, than that spontaneous generation occurred in this case and 
in no other ? 
He next tried if he could invent any reasonable hypothesis 
to account for the development of bacteria in these particular in- 
fusions, and finally arrived at the following :—Ordinary seeds— 
such as those of the common pea or bean, for instance—are, 
when fresh, easily boiled to a pulp and killed, but when 
thoroughly dried will bear prolonged boiling without becoming 
softened ; there are some seeds, indeed, which horticulturists 
regularly boil for some time before sowing. Why should not 
the same thing occur with the spores of Bacteria: was it not 
conceivable that prolonged exposure to a dry atmosphere—as 
would be the case in spores adhering to old hay—should so shrivel 
and harden these minute germs that even long application of 
the boiling temperature should be insufficient to kill them ? 
Having made the hypothesis, the next thing was to test its 
truth by making deductions from it and finding if these deduc- 
tions were in accordance with fact ; 2.2. to apply the method of 
verification. He said: If the hypothesis is true, the time taken to 
soften any spore will be roughly proportional to its age. Let us 
suppose our fluid to contain germs of very various ages. Justas if 
we had a bushel of peas of all degrees of dryness, a short boiling 
would completely soften the freshest, while it would hardly affect 
the driest; so in this case. A short boiling will kill all adult 
bacteria and perfectly fresh spores, but will only begin to soften 
older and drier spores. If, now, the fluid is placed aside for some 
hours, the least hardened of the unkilled spores will be softened 
and made ready to germinate: application of the boiling tempera- 
ture at this stage will kill them and begin to soften the next 
oldest. In this way, the application of heat at intervals for seve- 
ral days will kill the oldest and driest spores, since at the end of 
that time even these will be softened and ready to germinate. 
Tyndall verified this deduction by performing hundreds of new 
experiments on old hay infusion, and found that in every case 
the most obstinate infusion had all its germs infallibly killed by: 
this method of “ discontinuous heating.” 
On the whole, therefore, I think we may conclude that the 
results of recent research lend no support whatever to the 
doctrine of spontaneous generation. 
It must be borne in mind that in any case in which an organic 
infusion has remained unaltered for any length of time by being 
protected from atmospheric germs, the introduction of the smallest 
particle of a putrefying infusion or of the scum of a dried putre- 
faction will infallibly cause it to putrefy. 
It may be said of all I have told you so far, that it is doubtless 
very interesting from a scientific point of view, but that it is of no 
great practical importance. But the researches into this subject 
