of soda and ammonia presented, as did all the other tartrates, the
dissymmetry manifested by the absence of any plane of symmetry; that is
to say, the crystals of this salt placed before a mirror produced an
image which was not superposable upon the crystal. It was like a right
hand having its left for an image. With regard to the paratartrates of
soda and ammonia, one circumstance struck Pasteur in a quite unexpected
manner. Far from establishing in the crystals of this salt the absence
of all dissymmetry, he found that they all manifestly possessed it.
But, strange to say, certain crystals possessed it in one sense and
other crystals in a sense opposite. Some of these crystals, when
placed before a mirror, produced the image of the others, and one of
the two kinds of crystals corresponded rigorously in form with the
tartrate prepared by means of the tartaric acid of the grape. Pasteur
continued his reasoning thus:--Since there is no difference between
the form of the tartrate derived from the tartaric acid of the grape
and one of the two kinds of crystals deposited at the moment of
crystallisation of the paratartrate, the simple observation of the
dissymmetry proper to each will enable me to separate, by hand, all
the crystals of the paratartrate which are identical with those of
the tartrate. By ordinary chemical processes I ought to be able to
extract a tartaric acid identical with that of the grape, possessing
all its physical, mineralogical, and chemical properties--that is to
say, a tartaric acid possessing, like the natural tartaric acid of the
grape, dissymmetry of form, and exerting an action on polarised light.
_Per contra_, I ought to be able to extract from the second sort of
crystals, associated with the former in the paratartaric group, an
acid which will reproduce ordinary tartaric acid, but possessing a
dissymmetry of an inverse kind and exerting an action equally inverse
on polarised light.
With a feverish ardour Pasteur hastened to make this double experiment.
Imagine his joy when he saw his anticipations not only realised but
realised with an exactitude truly mathematical. His delight was so
great that he quitted the laboratory abruptly. Hardly had he gone
out when he met the assistant of the physical professor. He embraced
him, exclaiming, 'My dear Monsieur Bertrand, I have just made a great
discovery! I have separated the double paratartrate of soda and ammonia
into two salts of inverse dissymmetry, and exerting an inverse action
on the plane of polarisation of light. I am so happy that a nervous
tremulousness has taken possession of me, which prevents me from
looking again through the polariscope. Let us go to the Luxembourg, and
I will explain it all to you.'
These results excited in a high degree the attention of the Academy of
Sciences, where sat, at the time now referred to, Arago, Biot, Dumas,
De Senarmont, and Balard. It might be said without exaggeration that
the Academy was astounded. At the same time there were many members who
were slow to believe in this discovery. Charged with drawing up the
report, M. Biot began by requiring from Pasteur the verification of
each point which he had announced. To this verification M. Biot brought
his habitual precision, which was associated with a kind of suspicious
scepticism.
In one of his lectures Pasteur thus described his interview with M.
Biot:--'He made me come to his house, where he put into my hands
some paratartaric acid which he had carefully studied himself, and
found perfectly neutral as regards polarised light. It was not in the
laboratory of the École Normale, it was in his own kitchen, and in his
presence, that I was to prepare this double salt with soda and ammonia
procured by himself. The liquor was left slowly to evaporate, and at
the end of ten days, when it had deposited thirty or forty grammes of
crystals, he begged me to go over to the Collège de France to collect
the crystals and to extract from them specimens of the two kinds, which
he proposed to have placed, the one on his right hand, the other on
his left, desiring me to declare if I was ready to re-affirm, that
the crystals to the right would turn the plane of polarisation to
the right and the others to the left. This declaration made, he said
that he would charge himself with the rest of the inquiry. M. Biot
then prepared the solutions in well-measured proportions, and at the
moment of observing them in the polarising apparatus he invited me
again to come into his study. He placed first in the apparatus the
most interesting solution, that which ought to deviate to the left.
Without even making any measurements, he saw, by the mere inspection of
the colours of the ordinary and extraordinary images of the analyser,
that there was a strong deviation to the left. Visibly moved, the
illustrious old man took my arm and said, "My dear child, I have loved
science so well throughout my life that this makes my heart beat."'
The emotion of M. Biot was all the more profound because he had been
himself the first to discover the rotation of the plane of polarisation
by chemical substances, and had, for more than thirty years, affirmed
that the study of these substances and of their action in regard to
rotatory polarisation was, perhaps, the surest means of penetrating
into the intimate constitution of bodies. His counsels were received
with deference, but they had never been followed out. And now there
appeared before the old man, already somewhat discouraged, a youth
of twenty-five, who from his first investigation had proved himself
a master, who had dissipated the obscurities of the famous German
note, and created a new chapter in crystallographic chemistry. The
composition and nature of paratartaric acid had been explained, and
a new substance, the left-handed tartaric acid, with its truly
surprising properties, had been discovered; molecular physics and
chemistry had been enriched with new facts and theories of great value.
The first care of Pasteur, after having discovered the left-handed
tartaric acid and the constitution of paratartaric acid, was to compare
very carefully the properties of the new left-handed acid with those of
the right, endeavouring to determine by strict experiment the influence
on these properties of the internal atomic arrangements of the two
acids. Although we are unable to picture the exact figure of these
atomic groupings, there can be no doubt that they are formed of the
same elementary particles, that they are, moreover, dissymmetrical, and
that, in short, the dissymmetry of the one group is the same as that of
the other, but in an inverse sense. If, for example, the arrangement
of the atoms of the right-handed tartaric acid present the exterior
appearance of an irregular pyramid, the arrangement of the atoms of the
left-handed tartaric acid ought, of necessity, to present the form of a
pyramid irregular in the inverse sense.
II.
Nominated assistant professor of chemistry at Strasburg, Pasteur
followed up with enthusiasm these curious studies. To interrupt them
for an instant it required nothing less than his engagement with
Mademoiselle Marie Laurent, daughter of the Rector of the Academy.
It is even asserted that on the very morning of his marriage it was
necessary to go to his laboratory and remind him of the event that
was to take place on that day. But if Pasteur was thus guilty of an
absent-mindedness worthy of La Fontaine, he proved as a husband so
different from La Fontaine that Madame Pasteur, when reminded of this
lapse of memory, receives the reminder with an indulgent smile.
But to return to the laboratory: Under the same conditions of weight,
temperature, and quantity of solvent, Pasteur placed successively, in
presence of the two acids, all the substances capable of combining with
them. In this way he obtained right-handed and left-handed tartrates
of potash, of soda, of ammonia, of lime, and of all the oxides
properly so called. He applied himself to the compounds--and they are
numerous--which deposit themselves in liquids under well-determined
crystalline forms. Without entering into the details of these long
and patient studies, it may be stated generally that Pasteur proved
that whatever could be done with one of the tartaric acids could be
repeated rigorously, under similar conditions, with the other, the
resultant products manifesting constantly the same properties, with
the single difference already exhibited by the two acids--that in the
one case the deviation of the plane of polarisation was to the right,
while in the other it was to the left. With regard to all their other
properties, both chemical and physical, the identity was absolute.
Solubility, simple refraction by solutions, double refraction by
crystals, the action of heat in producing decomposition, &c., the
similitude extended to the most perfect identity.
The Academy of Sciences, which shows by the rarity of its reports the
importance which it attaches to them, gave for the second time an
account of these new researches. M. Biot was again the reporter. It was
with a sort of coquetry that Pasteur brought from Strasburg perfectly
labelled specimens of the magnificent crystallisations of the double
series of right-handed and left-handed tartrates. By means of models he
was able to render the forms of these crystals visible at a distance.
M. Biot undertook to bring the subject before the Academy. On the
morning of the day when he was to read his report he spent several
hours in conversation with Pasteur. M. Biot became so excited during
the discussion that Madame Biot, with the solicitude peculiar to the
wives of Academicians, requested Pasteur to change the subject of
conversation.
The members of the Academy shared the enthusiasm of M. Biot. Arago
moved that the report be inserted in the collected _mémoires_ of the
Academy. This was an exceptional honour. Arrived for the most part at
the end of their own careers, these learned men observed with pleasure
the incipient ray which had not yet become a glory but which was the
precursor thereof.
* * * * *
'My young friend,' said M. Biot to Pasteur, when presenting him to
Mitscherlich somewhere about that time, 'you may boast of having done
something great, in having discovered what had escaped such a man as
this.'
'I had studied,' replied Mitscherlich, not without a shade of regret,
addressing himself to Pasteur, 'I had studied with so much care and
perseverance, in their smallest details, the two salts which formed the
subject of my note to the Academy, that, if you have established what I
was unable to discover, you must have been guided to your result by a
preconceived idea.'
Mitscherlich was right, and this preconceived idea might have been
formulised thus: A dissymmetry in the internal molecular arrangement
of a chemical substance ought to manifest itself in all its external
properties which are themselves capable of dissymmetry.
* * * * *
If this theoretic conception was correct, Pasteur might expect to
find that all the substances in which M. Biot had observed the power
of rotating the plane of polarisation would possess the crystalline
dissymmetry revealed by the absence of superposability. The result was
in great part conformable to those previsions. The substances which
acted upon polarised light, as liquids or solutions, were generally
found by Pasteur to produce dissymmetric crystals. Some of them,
however, notwithstanding their power of crystallisation, exhibited,
when crystallised, no dissymmetric face. This difficulty did not deter
Pasteur. It gave him, on the contrary, the opportunity of showing that
when a theory had in so many cases proved itself correct, an apparent
objection must not be assumed insuperable without first sounding it to
the bottom. May it not be, he reasoned, that the absence of dissymmetry
in substances which have the molecular rotatory power is not an
accident; and may it not be possible, by changing the conditions of the
crystallisation, to make the dissymmetry appear?
Then, in order to modify the crystalline forms of substances which
did not show themselves to be spontaneously dissymmetrical, Pasteur
employed a method which had been often tried before, though its
principles could not be explained or its effects foreseen. In imitation
of Romé de Lisle, Leblanc, and Beudant, he varied the nature of his
solvents; he introduced into the solution, sometimes an excess of acid
or of base, sometimes foreign matters incapable of acting chemically
upon those which were to be modified; he even employed sometimes impure
mother liquids. On each occasion new facets were thus produced, and
these new facets showed the kind of dissymmetry which the optical
character demanded. Although he had to limit his researches to those
substances which, by their ready crystallisation and the beauty of
their forms, lent themselves best to this class of proofs, the results
were so far in accord with the previsions of theory, that no reasonable
doubt could exist as to the necessary correlation between dissymmetry
and the power to deviate polarised light.
* * * * *
By these researches Pasteur was led to a conclusion, which is worthy
of the most serious consideration, regarding the difference which
exists between mineral species and artificial products on the one
side, and the organic products which can be extracted from vegetables
or animals on the other. All mineral or artificial products--for
brevity let us say all the products of inorganic nature--have a
superposable image, and are therefore not dissymmetrical, while
vegetable and animal products--in other words, products formed under
the influence of life--have an image not superposable; that is to say,
they are atomically dissymmetrical, this dissymmetry expressing itself
externally in the power of turning the plane of polarisation. If any
exceptions exist they are more apparent than real. Pasteur himself
pointed out some of them, while demonstrating at the same time that
it is easy to explain why all trace of dissymmetry disappears when
substances which, like rock crystal, have an external dissymmetry are
subjected to the process of solution.
An apparent contradiction to this law of demarcation between artificial
products and those of animal and vegetable life is presented by the
existence in living creatures of substances like oxalic acid, formic
acid, urea, uric acid, creatine, &c. None of these products exert an
action on polarised light or show any dissymmetry in the form of their
crystals. But it is necessary to observe that these products are the
result of secondary actions. Their formation is evidently governed by
the laws which determine the constitution of the artificial products
of our laboratories, or of the mineral kingdom properly so called.
In living beings they are the products of excretion rather than
substances essential to vegetable or animal life. When, on the other
hand, we consider the most primordial substances of vegetables and
animals--those whereof it may be justly said that they are born under
the directive influence of _becoming_ life, such as cellulose, fecula,
albumen, fibrine, &c.--they are found to possess the power of acting,
on polarised light, a characteristic necessary and sufficient to
establish their internal dissymmetry, even when, through the absence of
crystallising power, they fail to manifest this dissymmetry outwardly.
It is, therefore, true to say that the products of inorganic nature,
whether mineral or artificial, have never yet presented molecular
dissymmetry. It may also be affirmed that the substances which exert
the greatest influence in vital manifestations, which are present and
active in the seed and in the egg at the moment of the marvellous start
of animal and vegetable life, all present molecular dissymmetry.
Would it be possible to indicate a more profound distinction between
the respective products of living and of mineral nature, than the
existence of this dissymmetry on the part of the one and its absence
on the part of the other? Is it not strange that not one of these
thousands and thousands of artificial products of the laboratory, the
number of which is each day augmented, should manifest either the power
of turning the plane of polarisation or non-superposable dissymmetry?
No doubt natural dissymmetric substances--gum, sugar, tartaric and
malic acids, quinine, strychnine, essence of turpentine, &c.--may be
employed in forming new compounds which remain dissymmetric, though
they are artificially prepared; but it is evident that all these new
products do but inherit the original dissymmetry of the substances from
which they are derived. When chemical action becomes more profound, all
dissymmetry disappears, and is never seen to reappear in the successive
ulterior products.
What can be the causes of so great a difference? M. Pasteur has often
expressed to me the conviction that it must be attributed to the
circumstance that the molecular forces which operate in the mineral
kingdom, and which are brought into play every day in our laboratories,
are forces of the symmetrical order; while the forces which are
present and active at the moment when the grain sprouts, when the egg
develops, and when, under the influence of the sun, the green matter
of the leaves decomposes the carbonic acid of the air and utilises in
divers ways the carbon of this acid, the hydrogen of the water, and the
oxygen of these two products--are of the dissymmetric order, probably
depending on some of the grand, dissymmetric, cosmic phenomena of our
universe. While expounding this opinion before the Academy of Sciences,
Pasteur, on one occasion, expressed himself thus:--
'The universe is a dissymmetric whole. I am inclined to think that
life, as manifested to us, must be a function of the dissymmetry of the
universe or of the consequences that follow in its train. The universe
is dissymmetrical; for, placing before a mirror the group of bodies
which compose the solar system, with their proper movements, we obtain
in the mirror an image not superposable on the reality. Even the motion
of solar light is dissymmetrical. A luminous ray never strikes in a
straight line, and at rest, the leaf wherein organic matter is created
by vegetable life. Terrestrial magnetism, the opposition which exists
between the north and south poles of a magnet, the opposition presented
to us by positive and negative electricity, are all the resultants of
dissymmetric actions and motions.'
* * * * *
At the moment when Pasteur, entering upon the labours which form the
principal subject of this book, abandoned the study of molecular
physics and chemistry which had previously occupied him, all his
thoughts were directed to the search of means suited to render evident
the influence of these causes and these phenomena. At Strasburg he
had procured powerful magnets with the view of comparing the actions
of their poles, and, if possible, of introducing by their aid, among
the forms of crystals, a manifestation of dissymmetry. At Lille, where
he was nominated Dean of the Faculty of Sciences in 1854, he had
contrived a piece of clockwork intended to keep a plant in continual
rotary motion, first in one direction and then in the other. 'All
this was gross,' he said to me one day; 'but, further than this, I
had proposed, with the view of influencing the vegetation of certain
plants, to invert, by means of a heliostat and a reflecting mirror,
the motion of the solar rays which should strike them from the birth
of their earliest shoots, and in this direction there was more to
be hoped for.' He never spoke of these attempts, because he had not
had the time to follow them to the issues of which he dreamed; but to
this day he remains persuaded that the barrier which exists between
the mineral and organic kingdoms--and which is revealed to our eyes by
the impossibility of producing, in the reactions of the laboratory,
dissymmetric organic substances--can never be crossed until we have
succeeded in introducing among these reactions influences of the
dissymmetric order. According to Pasteur, success in this direction
would give access to a new world of substances, and probably also of
organic transformations. As we have succeeded in finding the inverse
of right-handed tartaric acid, we may hope to obtain some day all the
immediate principles inverse to those now known to us. Who could say
what vegetable and animal species would become if it were possible to
replace, in the living cells, cellulose, albumen, and their congeners,
by their isomers with an inverse action? Certainly the thing is not
easy, and Pasteur would be the last person to deceive himself as to
the difficulty of the problem. His latest thought on the matter is
this:--When the attempt is made to introduce into living species
primordial substances, inverse to those now existing, the great
difficulty will be to master the _tendency_ (_devenir_[7]) proper to
the species, a tendency which is potential in the germ of each of them.
In this germ, it is to be feared, the dissymmetry of the dissymmetric
primordial substances which it embraces will always manifest itself.
Ah! if spontaneous generation were possible; if we could form from
mineral matter a living cell, how much more accessible would the
problem become! However this may be, we must seek, by all possible
means, to produce molecular dissymmetry by the application of forces
which have a dissymmetric action. 'We must,' said Pasteur to me on the
day when, starting from the note of Mitscherlich, he passed all these
things in review, 'we must invoke the action of solenoid or helix.
Entangled at present in labours more than sufficient to absorb whatever
of ardour and of force still remains to me, I have no longer time to
occupy myself with these questions.' But what great things are to be
done in following out this order of ideas, and what a route will be
opened to young men possessed of that genius of invention which is
evoked so often by persistent work!
* * * * *
This complete opposition between artificial mineral products and
vegetable and animal ones was to Pasteur a truth so well established
that he found frequent opportunity of affirming it under decisive
circumstances. One day, a very skilful chemist, M. Dessaignes,
who later on became one of the correspondents of the Academy of
Sciences, announced that he had transformed fumaric and malic acids
into aspartic acid. Pasteur, who some time previously had had
occasion to study these same acids, had proved that the two first
had no molecular dissymmetry--that is to say, they exercised no
optic action. In the state of solution they did not turn the plane
of polarised light. Aspartic acid, on the contrary, had presented to
him molecular dissymmetry, like asparagine itself. If the observation
of M. Dessaignes were true, then bodies which were inert in regard
to polarised light, and consequently non-dissymmetric, could be
transformed in the laboratory into active dissymmetric bodies. The
line of demarcation so well established would be broken. Pasteur,
whose experience regarding the note of Mitscherlich had shown him how
even the most conscientious observers may fail to seize upon fugitive
appearances, when unprompted to seek them by a preconceived idea,
doubted at once the accuracy of the facts cited by M. Dessaignes. From
Strasburg he started for Vendôme, where M. Dessaignes at that time
resided. M. Dessaignes immediately gave Pasteur a small quantity of
the aspartic acid which he had prepared by means of fumaric and malic
acids. Returning to his laboratory, Pasteur immediately recognised
that, despite the very close resemblance of the new acid of M.
Dessaignes to that derived from asparagine, the former differed from
the latter by the complete absence in its case of molecular dissymmetry.
With regard to other facts of the same kind, announced not only in
France, but in Italy, and in England--chiefly the pretended formation
of grape tartaric acid from succinic acid, artificial and inert,
by Perkin and Duppa--Pasteur testified with absolute certainty of
judgment to the existence of phenomenal peculiarities proper to these
substances, which he had never seen, and which had, on the other hand,
been the object of careful study by observers of great talent.