| Tracts on the nature of animals
and vegetables; observations and experiments upon the animalcula of infusions
1799 Lazaro Spallanzani
| Tracts on the nature of animals and vegetables. By Lazaro Spallanzani,
R. P. U. P., Edinburgh, 1799. Translation from the Italian. Translator?
Observations and experiments upon the animalcula of infusions, pages 149. |
I HERMETICALLY SEALED VESSELS WITH the eleven kinds of seeds mentioned
before [kidney beans, vetches, buckwheat, barley, maize, mallow, beet,
peas, lentils, beans, hemp]. To prevent the rarefaction of the internal
air, I diminished the thickness of the necks of the vessels, till they
terminated in tubes almost capillary, and, putting the smallest part to
the blowpipe, sealed it instantaneously, so that the internal air underwent
no alteration. It was necessary to know whether the seeds might suffer
by this inclusion, which might be an obstacle to the production of animalcula.
Other experiments had shewn me, 1. vessels hermetically sealed have no
animalcula, unless they are very capacious: 2. animalcula are not always
produced: 3. when they are produced, the number is never so great as in
open vessels. Although I used pretty large vessels, two substances, peas
and beans, had not a single animalcule. The other nine afforded a sufficient
number; and to these I limited my experiments. I took nine vessels with
seeds, hermetically sealed. I immersed them in boiling water for half a
minute. I immersed another nine for a whole minute, nine more for a minute
and a half, and nine for two minutes. Thus, I had thirty-six infusions.
That I might know the proper time to examine them, I made similar infusions
in open vessels, and, when these swarmed with animalcula, I opened those
hermetically sealed. Upon breaking the seal of the first, I found the elasticity
of the air increased. Seeds contain much air: a great quantity should escape
in their dissolution, by heat or maceration, which must, of necessity,
render the portion of included air denser and more elastic. However, the
elasticity may originate partly from the elastic fluid discovered in vegetables,
the nature of which is apparently different from the atmospheric fluid.
I examined the infusions, and was surprised to find some of them an absolute
desert; others reduced to such a solitude, that but a few animalcula, like
points, were seen, and their existence could be discovered only with the
greatest difficulty. The action of heat for one minute, was as injurious
to the production of the animalcula, as of two. The seeds producing the
inconceivably small animalcula, were, beans, vetches, buckwheat, mallows,
maize, and lentils. I could never discover the least animation in the other
three infusions. I thence concluded, that the heat of boiling water for
half a minute, was fatal to all animalcula of the largest kind; even to
the middle-sized, and the smallest, of those which I shall term animalcula
of the higher class, to use the energetic expression of M. Bonnet; while
the heat of two minutes did not affect those I shall place in the lower
class.
Having hermetically sealed six vessels, containing six kinds of seeds
producing animalcula of the lower class, I immersed them in boiling water
for two minutes and a half, three, three and a half, and four minutes.
The seals of twenty-four vessels being broken at a suitable time, there
were no animalcula of the higher class seen, but more or fewer of the lower.
The air was almost always condensed, both in this and in the other experiments.
In vessels immersed seven minutes, I found animalcula of the lower
class. They appeared in vessels immersed twelve minutes.
The minuteness of animalcula of the lower class, does not prevent our
distinguishing the difference of their figure and proportions.
Boiling half an hour was no obstacle to the production of animalcula
of the lower class; but boiling for three quarters, or even less, deprived
all the six infusions of animalcula.
We know, that the heat of boiling water is about 212°. These infusions
were of this heat at least, as appeared by the marks they exhibited of
ebullition, the whole time the surrounding water boiled. Philosophers know,
that water, boiled in a close vessel, acquires a greater degree of heat,
than when boiled in an open. To know how much less than half a minute the
boiling
might be abridged, an animalcula of the higher class yet exist, I made
use of a second-pendulum, and immersed the vessels in boiling water for
a given number of seconds, beginning with 29. In a word, boiling for a
single second prevented their existence; and I could only employ a degree
of heat less than that of boiling water.... Not a single animalcule was
seen of the higher class, in vessels hermetically sealed, and exposed to
the moderate heat of 113 ¡. This was during the middle of April:
the thermometer in the shade stood at 88°. I took eighteen vessels;
nine had been exposed to 99° of heat, and nine to 88°. No animalcula
of the higher class were produced in the former; but I found them in the
latter. In each vessel, the quantity and kind of animalcula, as in vessels
not subjected to heat. The degree of heat fatal to them, was 92°.
Animalcula of the lower class, exist in sealed vessels exposed to the
heat of 212°; while those of the higher class hardly appear at 92°:
But, when produced, the same intensity of heat that is fatal to the one,
also deprives the other of life; and animalcula of the higher, as well
as of the lower class, perish at 106°, or at most at 108°.
Two important consequences thence arise. The first evinces the extreme
efficacy of heat to deprive infusions in close vessels of a multitude of
animated beings; for, in open vessels, are always seen a vast concourse
of animalcula. The second consequence, concerns the constancy of animalcula
of the lower class appearing in infusions boiled in close vessels; and
the heat of 212¡, protracted an hour, has been no obstacle to their
existence....
We are therefore induced to believe, that those animalcula originate
from germs there included, which, for a certain time, withstand the effects
of heat, but at length yield under it; and, since animalcula of the higher
classes only exist when the heat is less intense, we must imagine they
are much sooner affected by it, than those of the lower classes. Whence
we should conclude, that this multitude of the superior animalcula, seen
in the infusions of open vessels, exposed not only to the heat of boiling
water, but to the flame of a blowpipe, appears there, not because their
germs have withstood so great a degree of heat, but because new germs come
to the infusions, after cessation of the heat....
In my observations, I have particularly inquired, whether animalcula
specifically varied as the infusions of vegetable seeds were different,
so that each might have peculiarly its own; but I have found nothing constant.
It is true, I have often found certain species of animals only, in particular
kinds of vegetables, but I have frequently seen the reverse. The animalcula
of the same infusion were different, at different times and different places;
and it even is not uncommon to see this variety in two infusions made of
the seed of the same plant. All this well agrees with the vast variety
of animalcular eggs, scattered in the air, and falling every where, without
any law.... The idea, that animalcula come from the air, appears to me
to be confirmed by undoubted facts. I took sixteen large and equal glass
vases: four I sealed hermetically; four were stopped with a wooden stopper,
well fitted; four with cotton; and the four last I left open. In each of
the four classes of vases, were hempseed, rice, lentils, and peas. The
infusions were boiled a full hour, before being put into the vases. I begun
the experiments 11. May, and visited the vases 5. June. In each there were
two kinds of animalcula, large and small; but in the four open ones, they
were so numerous and confused that the infusions, if I may use the expression,
rather seemed to teem with life. In those stoppered with cotton, they were
about a third more rare; still fewer in those with wooden stoppers; and
much more so in those hermetically sealed. . . .
The number of animalcula developed, is proportioned to the communication
with the external air. The air either conveys the germs to the infusions,
or assists the expansion of those already there....
Comment by Thomas Brock
Spallanzani's genius was in devising experiments that would be able
to answer the questions that were the most often raised by proponents of
spontaneous generation. His first experiment was to determine whether heating
really was able to kill all of the organisms within an infusion. To do
this, what simpler way than to set up a large series of flasks and heat
different ones for different lengths of time? When this was done, he discovered
that one group of microorganisms, which he calls superior animalcula, or
animalcula of the higher class, were destroyed by very slight heating.
These microorganisms were undoubtedly protozoa. The other group, which
he calls animalcula of the lower class, were much more resistant to heating.
From their description as being very minute, there is no doubt that these
were bacteria. Thus Spallanzani has shown that certain types of microorganisms
are much more sensitive to heat than others, and that for certain types
to be killed, the liquids must be heated to boiling for close to an hour.
Such an experiment is important in establishing the conditions necessary
for srudying spontaneous generation. Obviously, if the flasks are not heated
long enough, microorganisms might appear, and if one were not aware of
Spallanzani's experiments, it might be assumed that spontaneous generation
had occurred. Since he has shown that the superior animalcula are very
easily killed by heat, he can only infer that any superior animalcula appearing
in flasks which have been boiled and then allowed to stand exposed to the
air have come into the flasks from the air. This is a very important inference
which he then proceeds to test experimentally. By sealing vessels
with closures of varying degrees of porosity, he was able to show that
the
number of organisms present was a function of the porosity. Spallanzani
was
careful not to conclude that his experiment proved that microorganisms
were present in the air, since he also states that the air might be helping
those already present to grow. But the experiment was probably sufficient
to get people thinking about the possibility that microorganisms were present
in the air. This eventually stimulated Pasteur to devise a more critical
experiment which answered the question and became one of the turning points
in the controversy over spontaneous generation (see page 43). |