Discourses - Thomas H. Huxley
Pages:
1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23
When this action has continued, with more or less violence, for a certain
time, it gradually moderates. The evolution of bubbles slackens, and
finally comes to an end; scum and lees alike settle at the bottom, and
the fluid is once more clear and transparent. But it has acquired
properties of which no trace existed in the original liquid. Instead of
being a mere sweet fluid, mainly composed of sugar and water, the sugar
has more or less completely disappeared; and it has acquired that
peculiar smell and taste which we call "spirituous." Instead of being
devoid of any obvious effect upon the animal economy, it has become
possessed of a very wonderful influence on the nervous system; so that in
small doses it exhilarates, while in larger it stupefies, and may even
destroy life.
Moreover, if the original fluid is put into a still, and heated
moderately, the first and last product of its distillation is simple
water; while, when the altered fluid is subjected to the same process,
the matter which is first condensed in the receiver is found to be a
clear, volatile substance, which is lighter than water, has a pungent
taste and smell, possesses the intoxicating powers of the fluid in an
eminent degree, and takes fire the moment it is brought in contact with a
flame. The Alchemists called this volatile liquid, which they obtained
from wine, "spirits of wine," just as they called hydrochloric acid
"spirits of salt," and as we, to this day, call refined turpentine
"spirits of turpentine." As the "spiritus," or breath, of a man was
thought to be the most refined and subtle part of him, the intelligent
essence of man was also conceived as a sort of breath, or spirit; and, by
analogy, the most refined essence of anything was called its "spirit."
And thus it has come about that we use the same word for the soul of man
and for a glass of gin.
At the present day, however, we even more commonly use another name for
this peculiar liquid--namely, "alcohol," and its origin is not less
singular. The Dutch physician, Van Helmont, lived in the latter part of
the sixteenth and the beginning of the seventeenth century--in the
transition period between alchemy and chemistry--and was rather more
alchemist than chemist. Appended to his "Opera Omnia," published in 1707,
there is a very needful "Clavis ad obscuriorum sensum referendum," in
which the following passage occurs.--
"ALCOHOL.--Chymicis est liquor aut pulvis summe subtilisatus, vocabulo
Orientalibus quoque, cum primis Habessinis, familiari, quibus _cohol_
speciatim pulverem impalpabilem ex antimonio pro oculis tingendis denotat
... Hodie autem, ob analogiam, quivis pulvis tenerior ut pulvis oculorum
cancri summe subtilisatus _alcohol_ audit, haud aliter ac spiritus
rectificatissimi _alcolisati_ dicuntur."
Similarly, Robert Boyle speaks of a fine powder as "alcohol"; and, so
late as the middle of the last century, the English lexicographer, Nathan
Bailey, defines "alcohol" as "the pure substance of anything separated
from the more gross, a very fine and impalpable powder, or a very pure,
well-rectified spirit." But, by the time of the publication of
Lavoisier's "Traite Elementaire de Chimie," in 1789, the term "alcohol,"
"alkohol," or "alkool" (for it is spelt in all three ways), which Van
Helmont had applied primarily to a fine powder, and only secondarily to
spirits of wine, had lost its primary meaning altogether; and, from the
end of the last century until now, it has, I believe, been used
exclusively as the denotation of spirits of wine, and bodies chemically
allied to that substance.
The process which gives rise to alcohol in a saccharine fluid is known
tones as "fermentation"; a term based upon the apparent boiling up or
"effervescence" of the fermenting liquid, and of Latin origin.
Our Teutonic cousins call the same process "gaehren," "gaesen," "goeschen,"
and "gischen"; but, oddly enough, we do not seem to have retained their
verb or their substantive denoting the action itself, though we do use
names identical with, or plainly derived from, theirs for the scum and
lees. These are called, in Low German, "gaescht" and "gischt"; in Anglo-
Saxon, "gest," "gist," and "yst," whence our "yeast." Again, in Low
German and in Anglo-Saxon there is another name for yeast, having the
form "barm," or "beorm"; and, in the Midland Counties, "barm" is the name
by which yeast is still best known. In High German, there is a third name
for yeast, "hefe," which is not represented in English, so far as I know.
All these words are said by philologers to be derived from roots
expressive of the intestine motion of a fermenting substance. Thus "hefe"
is derived from "heben," to raise; "barm" from "beren" or "baeren," to
bear up; "yeast," "yst," and "gist," have all to do with seething and
foam, with "yeasty" waves, and "gusty" breezes.
The same reference to the swelling up of the fermenting substance is seen
in the Gallo-Latin terms "levure" and "leaven."
It is highly creditable to the ingenuity of our ancestors that the
peculiar property of fermented liquids, in virtue of which they "make
glad the heart of man," seems to have been known in the remotest periods
of which we have any record. All savages take to alcoholic fluids as if
they were to the manner born. Our Vedic forefathers intoxicated
themselves with the juice of the "soma"; Noah, by a not unnatural
reaction against a superfluity of water, appears to have taken the
earliest practicable opportunity of qualifying that which he was obliged
to drink; and the ghosts of the ancient Egyptians were solaced by
pictures of banquets in which the wine-cup passes round, graven on the
walls of their tombs. A knowledge of the process of fermentation,
therefore, was in all probability possessed by the prehistoric
populations of the globe; and it must have become a matter of great
interest even to primaeval wine-bibbers to study the methods by which
fermented liquids could be surely manufactured. No doubt it was soon
discovered that the most certain, as well as the most expeditious, way of
making a sweet juice ferment was to add to it a little of the scum, or
lees, of another fermenting juice. And it can hardly be questioned that
this singular excitation of fermentation in one fluid, by a sort of
infection, or inoculation, of a little ferment taken from some other
fluid, together with the strange swelling, foaming, and hissing of the
fermented substance, must have always attracted attention from the more
thoughtful. Nevertheless, the commencement of the scientific analysis of
the phenomena dates from a period not earlier than the first half of the
seventeenth century.
At this time, Van Helmont made a first step, by pointing out that the
peculiar hissing and bubbling of a fermented liquid is due, not to the
evolution of common air (which he, as the inventor of the term "gas,"
calls "gas ventosum"), but to that of a peculiar kind of air such as is
occasionally met with in caves, mines, and wells, and which he calls "gas
sylvestre."
But a century elapsed before the nature of this "gas sylvestre," or, as
it was afterwards called, "fixed air," was clearly determined, and it was
found to be identical with that deadly "choke-damp" by which the lives of
those who descend into old wells, or mines, or brewers' vats, are
sometimes suddenly ended; and with the poisonous aeriform fluid which is
produced by the combustion of charcoal, and now goes by the name of
carbonic acid gas.
During the same time it gradually became evident that the presence of
sugar was essential to the production of alcohol and the evolution of
carbonic acid gas, which are the two great and conspicuous products of
fermentation. And finally, in 1787, the Italian chemist, Fabroni, made
the capital discovery that the yeast ferment, the presence of which is
necessary to fermentation, is what he termed a "vegeto-animal" substance;
that is, a body which gives of ammoniacal salts when it is burned, and
is, in other ways, similar to the gluten of plants and the albumen and
casein of animals.
These discoveries prepared the way for the illustrious Frenchman,
Lavoisier, who first approached the problem of fermentation with a
complete conception of the nature of the work to be done. The words in
which he expresses this conception, in the treatise on elementary
chemistry to which reference has already been made, mark the year 1789 as
the commencement of a revolution of not less moment in the world of
science than that which simultaneously burst over the political world,
and soon engulfed Lavoisier himself in one of its mad eddies.
"We may lay it down as an incontestable axiom that, in all the operations
of art and nature, nothing is created; an equal quantity of matter exists
both before, and after the experiment: the quality and quantity of the
elements remain precisely the same, and nothing takes place beyond
changes and modifications in the combinations of these elements. Upon
this principle the whole art of performing chemical experiments depends;
we must always suppose an exact equality between the elements of the body
examined and those of the products of its analysis.
"Hence, since from must of grapes we procure alcohol and carbonic acid, I
have an undoubted right to suppose that must consists of carbonic acid
and alcohol. From these premisses we have two modes of ascertaining what
passes during vinous fermentation: either by determining the nature of,
and the elements which compose, the fermentable substances; or by
accurately examining the products resulting from fermentation; and it is
evident that the knowledge of either of these must lead to accurate
conclusions concerning the nature and composition of the other. From
these considerations it became necessary accurately to determine the
constituent elements of the fermentable substances; and for this purpose
I did not make use of the compound juices of fruits, the rigorous
analysis of which is perhaps impossible, but made choice of sugar, which
is easily analysed, and the nature of which I have already explained.
This substance is a true vegetable oxyd, with two bases, composed of
hydrogen and carbon, brought to the state of an oxyd by means of a
certain proportion of oxygen; and these three elements are combined in
such a way that a very slight force is sufficient to destroy the
equilibrium of their connection."
After giving the details of his analysis of sugar and of the products of
fermentation, Lavoisier continues:--
"The effect of the vinous fermentation upon sugar is thus reduced to the
mere separation of its elements into two portions; one part is oxygenated
at the expense of the other, so as to form carbonic acid; while the other
part, being disoxygenated in favour of the latter, is converted into the
combustible substance called alkohol; therefore, if it were possible to
re-unite alkohol and carbonic acid together, we ought to form sugar."[1]
[Footnote 1: _Elements of Chemistry_. By M. Lavoisier. Translated by
Robert Kerr. Second Edition, 1793 (pp. 186-196).]
Thus Lavoisier thought he had demonstrated that the carbonic acid and the
alcohol which are produced by the process of fermentation, are equal in
weight to the sugar which disappears; but the application of the more
refined methods of modern chemistry to the investigation of the products
of fermentation by Pasteur, in 1860, proved that this is not exactly
true, and that there is a deficit of from 5 to 7 per cent of the sugar
which is not covered by the alcohol and carbonic acid evolved. The
greater part of this deficit is accounted for by the discovery of two
substances, glycerine and succinic acid, of the existence of which
Lavoisier was unaware, in the fermented liquid. But about 1-1/2 per cent.
still remains to be made good. According to Pasteur, it has been
appropriated by the yeast, but the fact that such appropriation takes
place cannot be said to be actually proved.
However this may be, there can be no doubt that the constituent elements
of fully 98 per cent. of the sugar which has vanished during fermentation
have simply undergone rearrangement; like the soldiers of a brigade, who
at the word of command divide themselves into the independent regiments
to which they belong. The brigade is sugar, the regiments are carbonic
acid, succinic acid, alcohol, and glycerine.
From the time of Fabroni, onwards, it has been admitted that the agent by
which this surprising rearrangement of the particles of the sugar is
effected is the yeast. But the first thoroughly conclusive evidence of
the necessity of yeast for the fermentation of sugar was furnished by
Appert, whose method of preserving perishable articles of food excited so
much attention in France at the beginning of this century. Gay-Lussac, in
his "Memoire sur la Fermentation,"[2] alludes to Appert's method of
preserving beer-wort unfermented for an indefinite time, by simply
boiling the wort and closing the vessel in which the boiling fluid is
contained, in such a way as thoroughly to exclude air; and he shows that,
if a little yeast be introduced into such wort, after it has cooled, the
wort at once begins to ferment, even though every precaution be taken to
exclude air. And this statement has since received full confirmation from
Pasteur.
[Footnote 2: _Annales de Chimie_, 1810.]
On the other hand, Schwann, Schroeder and Dutch, and Pasteur, have amply
proved that air may be allowed to have free access to beer-wort, without
exciting fermentation, if only efficient precautions are taken to prevent
the entry of particles of yeast along with the air.
Thus, the truth that the fermentation of a simple solution of sugar in
water depends upon the presence of yeast, rests upon an unassailable
foundation; and the inquiry into the exact nature of the substance which
possesses such a wonderful chemical influence becomes profoundly
interesting.
The first step towards the solution of this problem was made two
centuries ago by the patient and painstaking Dutch naturalist,
Leeuwenhoek, who in the year 1680 wrote thus:--
"Saepissime examinavi fermnentum cerevisiae, semperque hoc ex globulis per
materiam pellucidam fluitantibus, quarm cerevisiam esse censui, constare
observavi: vidi etiam evidentissime, unumquemque hujus fermenti globulum
denuo ex sex distinctis globulis constare, accurate eidem quantitate et
formae, cui globulis sanguinis nostri, respondentibus.
"Verum talis mihi de horum origine et formatione conceptus formabam;
globulis nempe ex quibus farina Tritici, Hordei, Avenae, Fagotritici, se
constat aquae calore dissolvi et aquae commisceri; hac, vero aqua, quam
cerevisiam vocare licet, refrigescente, multos ex minimis particulis in
cerevisia coadunari, et hoc pacto efficere particulam sive globulum, quae
sexta pars est globuli faecis, et iterum sex ex hisce globulis
conjungi."[3]
[Footnote 3: Leeuwenhoek, _Arcana Naturae Detecta._ Ed. Nov., 1721.]
Thus Leeuwenhoek discovered that yeast consists of globules floating in a
fluid; but he thought that they were merely the starchy particles of the
grain from which the wort was made, rearranged. He discovered the fact
that yeast had a definite structure, but not the meaning of the fact. A
century and a half elapsed, and the investigation of yeast was
recommenced almost simultaneously by Cagniard de la Tour in France, and
by Schwann and Kuetzing in Germany. The French observer was the first to
publish his results; and the subject received at his hands and at those
of his colleague, the botanist Turpin, full and satisfactory
investigation.
The main conclusions at which they arrived are these. The globular, or
oval, corpuscles which float so thickly in the yeast as to make it muddy,
though the largest are not more than one two-thousandth of an inch in
diameter, and the smallest may measure less than one seven-thousandth of
an inch, are living organisms. They multiply with great rapidity by
giving off minute buds, which soon attain the size of their parent, and
then either become detached or remain united, forming the compound
globules of which Leeuwenhoek speaks, though the constancy of their
arrangement in sixes existed only in the worthy Dutchman's imagination.
It was very soon made out that these yeast organisms, to which Turpin
gave the name of _Torula cerevisioe_, were more nearly allied to the
lower Fungi than to anything else. Indeed Turpin, and subsequently
Berkeley and Hoffmann, believed that they had traced the development of
the _Torula_ into the well-known and very common mould--the _Penicillium
glaucum_. Other observers have not succeeded in verifying these
statements; and my own observations lead me to believe, that while the
connection between _Torula_ and the moulds is a very close one, it is of
a different nature from that which has been supposed. I have never been
able to trace the development of _Torula_ into a true mould; but it is
quite easy to prove that species of true mould, such as _Penicillium_,
when sown in an appropriate nidus, such as a solution of tartrate of
ammonia and yeast-ash, in water, with or without sugar, give rise to
_Toruloe_, similar in all respects to _T. cerevisioe_, except that they
are, on the average, smaller. Moreover, Bail has observed the development
of a _Torula_ larger than _T. cerevisioe_, from a _Mucor_, a mould allied
to _Penicillium_.
It follows, therefore, that the _Toruloe_, or organisms of yeast, are
veritable plants; and conclusive experiments have proved that the power
which causes the rearrangement of the molecules of the sugar is
intimately connected with the life and growth of the plant. In fact,
whatever arrests the vital activity of the plant also prevents it from
exciting fermentation.
Such being the facts with regard to the nature of yeast, and the changes
which it effects in sugar, how are they to be accounted for? Before
modern chemistry had come into existence, Stahl, stumbling, with the
stride of genius, upon the conception which lies at the bottom of all
modern views of the process, put forward the notion that the ferment,
being in a state of internal motion, communicated that motion to the
sugar, and thus caused its resolution into new substances. And Lavoisier,
as we have seen, adopts substantially the same view. But Fabroni, full of
the then novel conception of acids and bases and double decompositions,
propounded the hypothesis that sugar is an oxide with two bases, and the
ferment a carbonate with two bases; that the carbon of the ferment unites
with the oxygen of the sugar, and gives rise to carbonic acid; while the
sugar, uniting with the nitrogen of the ferment, produces a new substance
analogous to opium. This is decomposed by distillation, and gives rise to
alcohol. Next, in 1803, Thenard propounded a hypothesis which partakes
somewhat of the nature of both Stahl's and Fabroni's views. "I do not
believe with Lavoisier," he says, "that all the carbonic acid formed
proceeds from the sugar. How, in that case, could we conceive the action
of the ferment on it? I think that the first portions of the acid are due
to a combination of the carbon of the ferment with the oxygen of the
sugar, and that it is by carrying off a portion of oxygen from the last
that the ferment causes the fermentation to commence--the equilibrium
between the principles of the sugar being disturbed, they combine afresh
to form carbonic acid and alcohol."
The three views here before us may be familiarly exemplified by supposing
the sugar to be a card-house. According to Stahl, the ferment is somebody
who knocks the table, and shakes the card-house down; according to
Fabroni, the ferment takes out some cards, but puts others in their
places; according to Thenard, the ferment simply takes a card out of the
bottom story, the result of which is that all the others fall.
As chemistry advanced, facts came to light which put a new face upon
Stahl's hypothesis, and gave it a safer foundation than it previously
possessed. The general nature of these phenomena may be thus stated:--A
body, A, without giving to, or taking from, another body B, any material
particles, causes B to decompose into other substances, C, D, E, the sum
of the weights of which is equal to the weight of B, which decomposes.
Thus, bitter almonds contain two substances, amygdalin and synaptase,
which can be extracted, in a separate state, from the bitter almonds. The
amygdalin thus obtained, if dissolved in water, undergoes no change; but
if a little synaptase be added to the solution, the amygdalin splits up
into bitter almond oil, prussic acid, and a kind of sugar.
A short time after Cagniard de la Tour discovered the yeast plant,
Liebig, struck with the similarity between this and other such processes
and the fermentation of sugar, put forward the hypothesis that yeast
contains a substance which acts upon sugar, as synaptase acts upon
amygdalin. And as the synaptase is certainly neither organized nor alive,
but a mere chemical substance, Liebig treated Cagniard de la Tour's
discovery with no small contempt, and, from that time to the present, has
steadily repudiated the notion that the decomposition of the sugar is, in
any sense, the result of the vital activity of the _Torula_. But, though
the notion that the _Torula_ is a creature which eats sugar and excretes
carbonic acid and alcohol, which is not unjustly ridiculed in the most
surprising paper that ever made its appearance in a grave scientific
journal,[4] may be untenable, the fact that the _Toruloe_ are alive, and
that yeast does not excite fermentation unless it contains living
_Toruloe_, stands fast. Moreover, of late years, the essential
participation of living organisms in fermentation other than the
alcoholic, has been clearly made out by Pasteur and other chemists.
[Footnote 4: "Das entraethselte Geheimniss der geistigen Gaehrung
(Vorlaenfige briefliche Mittheilung)" is the title of an anonymous
contribution to Woehler and Liebig's _Annalen der Pharmacie_ for 1839, in
which a somewhat Rabelaisian imaginary description of the organisation of
the "yeast animals" and of the manner in which their functions are
performed, is given with a circumstantiality worthy of the author of
_Gulliver's Travels_. As a specimen of the writer's humour, his account
of what happens when fermentation comes to an end may suffice. "Sobald
naemlich die Thiere keinen Zucker mehr vorfinden, so fressen sie sich
gegenseitig selbst auf, was durch eine eigene Manipulation geschieht;
alles wird verdant bis auf die Eier, welche unveraendert durch den
Darmkanal hineingehen; man hat zuletzt wieder gaehrungsfaehige Hefe,
naemlich den Saamen der Thiere, der uebrig bleibt."] However, it may be
asked, is there any necessary opposition between the so-called "vital"
and the strictly physico-chemical views of fermentation? It is quite
possible that the living _Torula_ may excite fermentation in sugar,
because it constantly produces, as an essential part of its vital
manifestations, some substance which acts upon the sugar, just as the
synaptase acts upon the amygdalin. Or it may be, that, without the
formation of any such special substance, the physical condition of the
living tissue of the yeast plant is sufficient to effect that small
disturbance of the equilibrium of the particles of the sugar, which
Lavoisier thought sufficient to effect its decomposition.
Platinum in a very fine state of division--known as platinum black, or
_noir de platine_--has the very singular property of causing alcohol to
change into acetic acid with great rapidity. The vinegar plant, which is
closely allied to the yeast plant, has a similar effect upon dilute
alcohol, causing it to absorb the oxygen of the air, and become converted
into vinegar; and Liebig's eminent opponent, Pasteur, who has done so
much for the theory and the practice of vinegar-making, himself suggests
that in this case--
"La cause du phenomene physique qui accompagne la vie de la plante reside
dans un etat physique propre, analogue a celui du noir de platine. Mais
il est essentiel de remarquer que cet etat physique de la plante est
etroitement lie avec la vie de cette plante."[5]
[Footnote 5: _Etudes sur les Mycodermes_, Comptes-Rendus, liv., 1862.]
Now, if the vinegar plant gives rise to the oxidation of alcohol, on
account of its merely physical constitution, it is at any rate possible
that the physical constitution of the yeast plant may exert a decomposing
influence on sugar.
But, without presuming to discuss a question which leads us into the very
arcana of chemistry, the present state of speculation upon the _modus
operandi_ of the yeast plant in producing fermentation is represented, on
the one hand, by the Stahlian doctrine, supported by Liebig, according to
which the atoms of the sugar are shaken into new combinations either
directly by the _Toruloe_, or indirectly, by some substance formed by
them; and, on the other hand, by the Thenardian doctrine, supported by
Pasteur, according to which the yeast plant assimilates part of the
sugar, and, in so doing, disturbs the rest, and determines its resolution
into the products of fermentation. Perhaps the two views are not so much
opposed as they seem at first sight to be.