Twenty-seven putrescible infusions, first in vacuo, and afterwards
supplied with the most invigorating air, have shown no sign of
putrefaction or of life. And as to the others, I almost shrink from
asking him whether the hayloft has rendered them spontaneously
generative. Is not the inference here imperative that it is not the
air of the loft--which is connected through a constantly open door
with the general atmosphere--but something contained in the air, that
has produced the effects observed? What is this something? A sunbeam
entering through a chink in the roof or wall, and traversing the air
of the loft, would show it to be laden with suspended dust particles.
Indeed the dust is distinctly visible in the diffused daylight. Can
it have been the origin of the observed life? If so, are we not bound
by all antecedent experience to regard these fruitful particles 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. 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 upwards. They were blacker
than the blackest smoke ever seen issuing from the funnel of a
steamer; and their resemblance to smoke was so perfect as to prompt
the conclusion that the apparently pure flame of the alcohol-lamp
required but a beam of sufficient intensity to reveal its clouds of
liberated carbon.
But is the blackness smoke? This question presented itself in a
moment, and was thus answered: A red-hot poker was placed underneath
the beam; from it the black wreaths also ascended. A large hydrogen
flame, which emits no smoke, was next employed, and it also produced
with augmented copiousness those whirling masses of darkness. Smoke
being out of the question, what is the blackness? It is simply that
of stellar space; that is to say, blackness resulting from the absence
from the track of the beam of all matter competent to scatter its
light. When the flame was placed below the beam, the floating matter
was destroyed in situ; and the heated air, freed from this matter,
rose into the beam, jostled aside the illuminated particles, and
substituted for their light the darkness due to its own perfect
transparency. Nothing could more forcibly illustrate the invisibility
of the agent which renders all things visible. The beam crossed,
unseen, the black chasm formed by the transparent air, while, at both
sides of the gap, the thick-strewn particles shone out like a luminous
solid under the powerful illumination. [Footnote: See Fragment: 'On
Dust and Disease', vol. i.]
*****
Supposing an infusion intrinsically barren, but readily susceptible of
putrefaction when exposed to common air, to be brought into contact
with this unilluminable air, what would be the result? It would never
putrefy. It might, however, be urged that the air is spoiled by its
violent calcination. Oxygen passed through a spirit-lamp flame is, it
may be thought, no longer the oxygen suitable for the development and
maintenance of life. We have an easy escape from this difficulty,
which is based, however, upon the unproved assumption that the air has
been affected by the flame. Let a condensed beam be sent through a
large flask or bolthead containing common air. The track of the beam
is seen within the flask--the dust revealing the light, and the light
revealing the dust. Cork the flask, stuff its neck with cotton-wool,
or simply turn it mouth downwards and leave it undisturbed for a day
or two. Examined afterwards with the luminous beam, no track is
visible; the light passes through the flask as through a vacuum. The
floating matter has abolished itself, being now attached to the
interior surface of the flask.
Were it our object, as it will be subsequently, to effectually detain
the dirt, we might coat that surface with some sticky substance. Here,
then, without 'torturing' the air in any way, we have found a means of
ridding it, or rather of enabling it to rid itself, of floating
matter.
We have now to devise a means of testing the action of such
spontaneously purified air upon putrescible infusions. Wooden
chambers, or cases, are accordingly constructed, having glass fronts,
side-windows, and back-doors. Through the bottoms of the chambers
test-tubes pass air-tight; their open ends, for about one-fifth of the
length of the tubes, being within the chambers. Provision is made for
a free connection rough sinuous channels between the inner and the
outer air. Through such channels, though open, no dust will reach the
chamber. The top of each chamber is perforated by a circular hole two
inches in diameter, closed air-tight by a sheet of India-rubber. This
is pierced in the middle by a pin, and through the pin-hole is pushed
the shank of a long pipette, ending above in a small funnel. The
shank also passes through a stuffing-box of cotton-wool moistened with
glycerine; so that, tightly clasped by the rubber and wool, the
pipette is not likely in its motions up and down to carry any dust
into the chamber. The annexed woodcut shows a chamber, with six
test-tubes, its side-windows w w, its pipette p c, and its sinuous
channels a b which connect the air of the chamber with the outer air.
The chamber is carefully closed and permitted to remain quiet for two
or three days. Examined at the beginning by a beam sent through its
windows, the air is found laden with floating matter, which in three
days has wholly disappeared. To prevent its ever rising again, the
internal surface of the chamber was at the outset coated with
glycerine. The fresh but putrescible liquid is introduced into the
six tubes in succession by means of the pipette. Permitted to remain
without further precaution, every one of the tubes would putrefy and
fill itself with life. The liquid has been in contact with the
dust-laden air outside by which it has been infected, and the
infection must be destroyed. This is done by plunging the six tubes
into a bath of heated oil and boiling the infusion. The time
requisite to destroy the infection depends wholly upon its nature. Two
minutes' boiling suffices to destroy some _contagia_, whereas two
hundred minutes' boiling fails to destroy others. After the infusion
has been sterilised, the oil-bath is withdrawn, and the liquid, whose
putrescibility has been in no way affected by the boiling, is
abandoned to the air of the chamber.
With such chambers I tested, in the autumn and winter of 1875-6,
infusions of the most various kinds, embracing natural animal liquids,
the flesh and viscera of domestic animals, game, fish, and vegetables.
More than fifty chambers, each with its series of infusions, were
tested, many of them repeatedly. There was no shade of uncertainty in
any of the results. In every instance we had, within the chamber,
perfect limpidity and sweetness, which in some cases lasted for more
than a year--without the chamber, with the same infusion, putridity
and its characteristic smells. In no instance was the least
countenance lent to the notion that an infusion deprived by heat of
its inherent life, and placed in contact with air cleansed of its
visibly suspended matter, has any power to generate life anew.
Remembering then the number and variety of the infusions employed, and
the strictness of our adherence to the rules of preparation laid down
by the heterogenists themselves; remembering that we have operated
upon the very substances recommended by them as capable of furnishing,
even in untrained hands, easy and decisive proofs of spontaneous
generation, and that we have added to their substances many others of
our own--if this pretended generative power were a reality, surely it
must have manifested itself somewhere. Speaking roundly, I should say
that in such closed chambers at least five hundred chances have been
given to it, but it has nowhere appeared.
The argument is now to be clenched by an experiment which will remove
every residue of doubt as to the ability of the infusions here
employed to sustain life. We open the back doors of our sealed
chambers, and permit the common air with its floating particles to
have access to our tubes. For three months they have remained
pellucid and sweet--flesh, fish, and vegetable extracts purer than
ever cook manufactured. Three days' exposure to the dusty air
suffices to render them muddy, fetid, and swarming with infusorial
life. The liquids are thus proved, one and all, ready for
putrefaction when the contaminating agent is applied. I invite my
colleague to reflect on these facts. How will he account for the
absolute immunity of a liquid exposed for months in a warm room to
optically pure air, and its infallible putrefaction in a few days when
exposed to dust-laden air? He must, I submit, bow to the conclusion
that the dust-particles are the cause of putrefactive life. And
unless he accepts the hypothesis that these particles, being dead in
the air, are in the liquid miraculously kindled into living things, he
must conclude that the life we have observed springs from germs or
organisms diffused through the atmosphere.
The experiments with hermetically sealed flasks have reached the
number of 940. A sample group of 130 of them were laid before the
Royal Society on January 13, 1876. They were utterly free from life,
having been completely sterilised by three minutes' boiling. Special
care had been taken that the temperatures to which the flasks were
exposed should include those previously alleged to be efficient. The
conditions laid down by the heterogenist were accurately copied, but
there was no corroboration of his results. Stress was then laid on
the question of warmth, thirty degrees being suddenly added to the
temperatures with which both of us had previously worked. Waiving all
protest against the caprice thus manifested, I met this new
requirement also. The sealed tubes, which had proved barren in the
Royal Institution, were suspended in perforated boxes, and placed
under the supervision of an intelligent assistant in the Turkish Bath
in Jermyn Street. From two to six days had been allowed for the
generation of organisms in hermetically sealed tubes. Mine remained
in the washing-room of the bath for nine days. Thermometers placed in
the boxes, and read off twice or three times a day, showed the
temperature to vary from a minimum of 101° to a maximum of 112° Fahr.
At the end of nine days the infusions were as clear as at the
beginning. They were then removed to a warmer position. A
temperature of 115° had been mentioned as particularly favourable to
spontaneous generation. For fourteen days the temperature of the
Turkish Bath hovered about this point, falling once as low as 106°,
reaching 116° on three occasions, 118° on one, and 119° on two. The
result was quite the same as that just recorded. The higher
temperatures proved perfectly incompetent to develope life.
Taking the actual experiment we have made as a basis of calculation,
if our 940 flasks were opened on the hayloft of the Bel Alp, 858 of
them would become filled with organisms. The escape of the remaining
82 strengthens our case, proving as it does conclusively that not in
the air, nor in the infusions, nor in anything continuous diffused
through the air, but in discrete particles, suspended in the air and
nourished by the infusions, we are to seek the cause of life. Our
experiment proves these particles to be in some cases so far apart on
the hayloft as to permit 10 per cent of our flasks to take in air
without contracting contamination. A quarter of a century ago Pasteur
proved the cause of 'so-called spontaneous generation' to be
discontinuous. I have already referred to his observation that 12 out
of 20 flasks opened on the plains escaped infection, while 19 out of
20 flasks opened on the Mer de Glace escaped. Our own experiment at
the Bel Alp is a more emphatic instance of the same kind, 90 per cent
of the flasks opened in the hayloft being smitten, while not one of
those opened on the free mountain ledge was attacked.
The power of the air as regards putrefactive infection is incessantly
changing through natural causes, and we are able to alter it at will.
Of a number of flasks opened in 1876 in the laboratory of the Royal
Institution, 42 per cent. were smitten, while 58 per cent. escaped.
In 1877 the proportion in the same laboratory was 68 per cent.
smitten, to 32 intact. The greater mortality, so to speak, of the
infusions in 1877 was due to the presence of hay which diffused its
germinal dust in the laboratory air, causing it to approximate as
regards infective virulence to the air of the Alpine loft. I would
ask my friend to bring his scientific penetration to bear upon all the
foregoing facts. They do not prove spontaneous generation to be
'impossible.' My assertions, however, relate not to 'possibilities,'
but to proofs, and the experiments just described do most 'distinctly
prove the evidence on which the heterogenist relies to be written on
waste paper.
My colleague will not, I am persuaded, dispute these results; but he
may be disposed to urge that other able and honourable men working at
the same subject have arrived at conclusions different from mine. Most
freely granted; but let me here recur to the remarks already made in
speaking of the experiments of Spallanzani, to the effect that the
failure of others to confirm his results by no means upsets their
evidence. To fix the ideas, let us suppose that my colleague comes to
the laboratory of the Royal Institution, repeats there my experiments,
and obtains confirmatory results; and that he then goes to University
or King's College where, operating with the same infusions, he obtains
contradictory results. Will he be disposed to conclude that the
selfsame substance is barren in Albemarle Street and fruitful in Gower
Street or the Strand? His Alpine experience has already made known to
him the literally infinite differences existing between different
samples of air as regards their capacity for putrefactive infection.
And, possessing this knowledge, will he not substitute for the
adventurous conclusion that an organic infusion is barren at one place
and spontaneously generative at another, the more rational and obvious
one that the atmospheres of the two localities which have had access
to the infusion are infective in different degrees?
As regards workmanship, moreover, he will not fail to bear in mind,
that fruitfulness may be due to errors of manipulation, while
barrenness involves the presumption of correct experiment. It is only
the careful worker that can secure the latter, while it is open to
every novice to obtain the former. Barrenness is the result at which
the conscientious experimenter, whatever his theoretic convictions may
be, ought to aim, omitting no pains to secure it, and resorting only
when there is no escape from it to the conclusion that the life
observed comes from no source which correct experiment could
neutralise or avoid.
Let us again take a definite case. Supposing my colleague to operate
with the same apparent care on 100 infusions--or rather on 100 samples
of the same infusion--and that 50 of them prove fruitful and 50
barren. Are we to say that the evidence for and against heterogeny is
equally balanced? There are some who would not only say this, but who
would treasure up the 50 fruitful flasks as 'positive' results, and
lower the evidential value of the 50 barren flasks by labelling them
'negative' results. This, as shown by Dr. William Roberts, is an
exact inversion of the true order of the terms positive and negative.
[Footnote: See his truly philosophical remarks on this head in the
'British Medical Journal,' 1876, p. 282.] Not such, I trust, would
be the course pursued by my friend. As regards the 50 fruitful flasks
he would, I doubt not, repeat the experiment with redoubled care and
scrutiny, and not by one repetition only, but by many, assure himself
that he had not fallen into error. Such faithful scrutiny fully
carried out would infallibly lead him to the conclusion that here, as
in all other cases, the evidence in favour of spontaneous generation
crumbles in the grasp of the competent enquirer.
The botanist knows that different seeds possess different powers of
resistance to heat. [Footnote: I am indebted to Dr. Thiselton Dyer for
various illustrations of such differences. It is, however, surprising
that a subject of such high scientific importance should not have been
more thoroughly explored. Here the scoundrels who deal in killed
seeds might be able to add to our knowledge.] Some are killed by a
momentary exposure to the boiling temperature, while others withstand
it for several hours. Most of our ordinary seeds are rapidly killed,
while Pouchet made known to the Paris Academy of Sciences in 1866,
that certain seeds, which had been transported in fleeces of wool from
Brazil, germinated after four hours' boiling. The germs of the air
vary as much among themselves as the seeds of the botanist. In some
localities the diffused germs are so tender that boiling for five
minutes, or even less, would be sure to destroy them all; in other
localities the diffused germs are so obstinate, that many hours'
boiling would be requisite to deprive them of their power of
germination. The absence or presence of a truss of desiccated hay
would produce differences as great as those here described. The
greatest endurance that I have ever observed--and I believe it is the
greatest on record--was a case of survival after eight hours' boiling.
As regards their power of resisting heat, the infusorial germs of our
atmosphere might be classified under the following and intermediate
heads: Killed in five minutes; not killed in five minutes but killed
in fifteen; not killed in fifteen minutes but killed in thirty; not
killed in thirty minutes but killed in an hour; not killed in an hour
but killed in two hours; not killed in two but killed in three hours;
not killed in three but killed in four hours. I have had several
cases of survival after four and five hours' boiling, some survivals
after six, and one after eight hours' boiling. Thus far has
experiment actually reached; but there is no valid warrant for fixing
upon even eight hours as the extreme limit of vital resistance.
Probably more extended researches (though mine have been very
extensive) would reveal germs more obstinate still. It is also
certain that we might begin earlier, and find germs which are
destroyed by a temperature far below that of boiling water. In the
presence of such facts, to speak of a death-point of bacteria and
their germs would be unmeaning--but of this more anon.
'What present warrant,' it has been asked, 'is there for supposing
that a naked, or almost naked, speck of protoplasm can withstand four,
six, or eight hours' boiling?' Regarding naked specks of protoplasm I
make no assertion. I know nothing about them, save as the creatures
of fancy. But I do affirm, not as a 'supposition,' nor an
'assumption,' nor a 'probable guess,' nor as 'a wild hypothesis,' but
as a matter of the most undoubted fact, that the spores of the hay
bacillus, when thoroughly desiccated by age, have withstood the ordeal
mentioned. And I further affirm that these obdurate germs, under the
guidance of the knowledge that they are germs, can be destroyed by
five minutes' boiling, or even less. This needs explanation. The
finished bacterium perishes at a temperature far below that of boiling
water, and it is fair to assume that the nearer the germ is to its
final sensitive condition the more readily will it succumb to heat.
Seeds soften before and during germination. This premised, the simple
description of the following process will suffice to make its meaning
understood.
An infusion infected with the most powerfully resistent germs, but
otherwise protected against the floating matters of the air, is
gradually raised to its boiling-point. Such germs as have reached the
soft and plastic state immediately preceding their development into
bacteria are thus destroyed. The infusion is then put aside in a warm
room for ten or twelve hours. If for twenty-four, we might have the
liquid charged with well-developed bacteria. To anticipate this, at
the end of ten or twelve hours we raise the infusion a second time to
the boiling temperature, which, as before, destroys all germs then
approaching their point of final development. The infusion is again
put aside for ten or twelve hours, and the process of heating is
repeated. We thus kill the germs in order of their resistance, and
finally kill the last of them. No infusion can withstand this process
if it be repeated a sufficient number of times. Artichoke, cucumber,
and turnip infusions, which had proved specially obstinate when
infected with the germs of desiccated hay, were completely broken down
by this method of discontinuous heating, three minutes being found
sufficient to accomplish what three hundred minutes' continuous
boiling failed to accomplish. I applied the method, moreover, to
infusions of various kinds of hay, including those most tenacious of
life. Not one of them bore the ordeal. These results were clearly
foreseen before they were realised, so that the germ theory fulfils
the test of every true theory, that test being the power of prevision.