RISE AND PROGRESS OF ORGANIC CHEMISTRY--PERIOD OF LIEBIG AND DUMAS.
_Justus Liebig, 1803-1873. Jean Baptiste Andre Dumas, born in 1800._
I have as yet said almost nothing with regard to the progress of organic
chemistry, considered as a special branch of the science. It is however in
this department that the greatest triumphs which mark the third period of
chemical advance have been won. We must therefore now turn our attention to
the work which has been done here.
The ancients drew no such distinction between portions of their chemical
knowledge, limited as it was, as is implied by the modern terms "organic"
and "inorganic chemistry." An organic acid--acetic--was one of the earliest
known substances belonging to the class of acids; many processes of
chemical handicraft practised in the olden times dealt with the manufacture
of substances, such as soap, leather or gum, which we should now call
organic substances. Nor did the early alchemists, although working chiefly
with mineral or inorganic substances, draw any strict division between the
two branches of chemistry. The medical chemists of the sixteenth century
dealt much with substances derived from plants and animals, such as benzoic
and succinic acids, spirit of wine, oils, etc. But neither in their
nomenclature nor in their practice did they sharply distinguish inorganic
from organic compounds. They spoke of the _quintessence_ of arsenic and the
_quintessence_ of alcohol; they applied the term "oil" alike to the
products of the action of acids on metallic salts and to substances
obtained from vegetables. But towards the end of the seventeenth century,
at the time that is when the phlogistic theory began to gain pre-eminence,
we find gradually springing up a division of chemical substances into
mineral, animal and vegetable substances--a division which was based rather
on a consideration of the sources whence the substances were derived than
on the properties of the substances themselves, and therefore a division
which was essentially a non-chemical one.
About a century after this, systematic attempts began to be made to trace
some peculiarity of composition as belonging to all compounds of organic,
that is, of animal or vegetable, origin. As very many of the substances
then known belonging to this class were more or less oil-like in their
properties--oils, fats, balsams, gums, sugar, etc.--organic substances
generally were said to be characterized by the presence in them of the
_principle of oil_.
Such a statement as this, although suited to the conceptions of that time,
could not be received when Lavoisier had shown chemists how Nature ought be
examined. With the definite conception of element introduced by the new
chemistry, came an attempt to prove that organic compounds were built up of
elements which were rarely found together in any one compound of inorganic
origin. Substances of vegetable origin were said by Lavoisier to be
composed of carbon, hydrogen and oxygen, while phosphorus and nitrogen, in
addition to those three elements, entered into the composition of
substances derived from animals. But neither could this definition of
organic compounds be upheld in the face of facts. Wax and many oils
contained only carbon and hydrogen, yet they were undoubtedly substances of
vegetable or animal origin. If the presence of any two of the three
elements, carbon, hydrogen and oxygen, were to be regarded as a sufficient
criterion for the classification of a compound, then it was necessary that
carbonic acid--obtained by the action of a mineral acid on chalk--should be
called an organic compound.
To Berzelius belongs the honour of being the chemist who first applied the
general laws of chemical combination to all compounds alike, whether
derived from minerals, animals, or vegetables. The ultimate particles, or
molecules, of every compound were regarded by Berzelius as built up of two
parts, each of which might itself be an elementary atom, or a group of
elementary atoms. One of these parts, he said, was characterized by
positive, the other by negative electricity. Every compound molecule,
whatever was the nature or number of the elementary atoms composing it, was
a dual structure (see p. 164). Organic chemistry came again to be a term
somewhat loosely applied to the compounds derived from animals or
vegetables, or in the formation of which the agency of living things was
necessary. Most, if not all of these compounds contained carbon and some
other element or elements, especially hydrogen, oxygen and nitrogen.
But the progress of this branch of chemistry was impeded by the want of any
trustworthy methods for analysing compounds containing carbon, oxygen and
hydrogen. This want was to be supplied, and the science of organic
chemistry, and so of chemistry in general, was to be immensely advanced by
the labours of a new school of chemists, chief among whom were Liebig and
Dumas.
Let us shortly trace the work of these two renowned naturalists. The
life-work of the first is finished; I write this story of the progress of
his favourite science on the eighty-second birthday of the second of these
great men, who is still with us a veteran crowned with glory, a true
soldier in the battle against ignorance and so against want and crime.
* * * * *
JUSTUS LIEBIG was born at Darmstadt, on the 12th of May 1803. The main
facts which mark his life regarded apart from his work as a chemist are
soon told. Showing a taste for making experiments he was apprenticed by his
father to an apothecary. Fortunately for science he did not long remain as
a concoctor of drugs, but was allowed to enter the University of Bonn as a
student of medicine. From Bonn he went to Erlangen, at which university he
graduated in 1821. A year or two before this time Liebig had begun his
career as an investigator of Nature, and he had already made such progress
that the Grand Duke of Hesse-Darmstadt was prevailed on to grant him a
small pension and allow him to prosecute his researches at Paris, which was
then almost the only place where he could hope to find the conditions of
success for the study of scientific chemistry. To Paris accordingly he went
in 1823. He was so fortunate--thanks to the good graces of the renowned
naturalist Alexander von Humboldt--as to be allowed to enter the laboratory
of Gay-Lussac, where he continued the research on a class of explosive
compounds, called _fulminates_, which he had begun before leaving
Darmstadt.
A year later Liebig was invited to return to his native country as
Professor of Chemistry in the small University of Giessen--a name soon to
be known wherever chemistry was studied, and now held dear by many eminent
chemists who there learned what is meant by the scientific study of Nature.
The year before Liebig entered the laboratory of Gay-Lussac there came to
Paris a young and enthusiastic student who had already made himself known
in the scientific world by his physiological researches, and who was now
about to begin his career as a chemist.
In that southern part of France which is rich in memories of the Roman
occupation, not far from the remains of the great aqueduct which spans the
valley of the Gardon, at no great distance from the famous cities of Arles
and Nimes, was born, in the town of Alais, on the 14th of July 1800, JEAN
BAPTISTE ANDRE DUMAS.
The father of Dumas was a man of considerable culture; he gave his son as
good an education as could be obtained in the little town of his birth. At
the age of fourteen young Dumas was a good classical scholar, and had
acquired a fair knowledge of natural science. But for his deficiency in
mathematics he would probably have entered for the examination which
admitted those who passed it to join the French navy. But before he had
made good his mathematical deficiencies the troublous nature of the times
(1814-15) obliged his parents to think of some other profession for their
son which would entail less sacrifice on their part.
Like his great fellow-worker in after life he was apprenticed to an
apothecary, and like him also, he soon forsook this sphere of usefulness.
Desirous of better opportunities for the study of science, and overpowered
by the miseries which war had brought upon the district of his birth, Dumas
persuaded his father to allow him to go to Geneva. At Geneva Dumas found an
atmosphere more suited to his scientific progress; chemistry, physics,
botany, and other branches of natural science were taught by men whose
names were everywhere known. He began experiments in chemistry with the
crudest and most limited apparatus, but even with these he made discoveries
which afterwards led to important work on the volumes occupied by the atoms
of elementary substances.
About the year 1818 Dumas became acquainted with Dr. J. L. Prevost, who had
returned from studying in many of the most famous medical schools of
Europe. Invited by Prevost to join in an investigation requiring medical,
botanical and chemical knowledge, Dumas now began a series of researches
which soon passed into the domain of animal physiology, and by the
prosecution of which under many difficulties he laid the foundations of his
future fame.
But along with his physiological work Dumas carried on a research into the
expansion of various ethers. This necessitated the preparation of a series
of ethers in a state of purity; but so difficult did Dumas find this to be,
so much time did he consume in this preliminary work, and so interested
did he become in the chemical part of the investigation, that he abandoned
the experiments on expansion, and set himself to solve some of the problems
presented by the composition and chemical properties of the ethers.
Dumas would probably have remained in Geneva had he not had a morning visit
paid him in the year 1822. When at work in his laboratory one day, some one
knocked and was bidden come in. "I was surprised to find myself face to
face with a gentleman in a light-blue coat with metal buttons, a white
waistcoat, nankeen breeches, and top-boots.... The wearer of this costume,
his head somewhat bent, his eyes deep-set but keen, advanced with a
pleasant smile, saying, 'Monsieur Dumas.' 'The same, sir; but excuse me.'
'I am M. de Humboldt, and did not wish to pass through Geneva without
having had the pleasure of seeing you.'... I had only one chair. My visitor
was pleased to accept it, whilst I resumed my elevated perch on the drawing
stool.... 'I intend,' said M. de Humboldt, 'to spend some days in Geneva,
to see old friends and to make new ones, and more especially to become
acquainted with young people who are beginning their career. Will you act
as my cicerone? I warn you however that my rambles begin early and end
late. Now, could you be at my disposal, say from six in the morning till
midnight?'" After some days spent as Humboldt had indicated the great
naturalist left Geneva. Dumas tells us that the town seemed empty to him.
"I felt as if spell-bound. The memorable hours I had spent with that
irresistible enchanter had opened a new world to my mind." Dumas felt that
he must go to Paris--that there he would have more scope and more
opportunities for prosecuting science. A few kind words, a little genuine
sympathy, and a little help from Humboldt were thus the means of fairly
launching in their career of scientific inquiry these two young men, Liebig
and Dumas.
In Paris, whither he went in 1823, Dumas found a welcome. He soon made the
acquaintance and gained the friendship of the great men who then made
natural science so much esteemed in the French capital. When the year 1826
came, it saw him Professor of Chemistry at the Athenaeum, and married to the
lady whom he loved, and who has ever since fought the battle of life by his
side.
Liebig left Paris in 1824. By the year 1830 he had perfected and applied
that method for the analysis of organic compounds which is now in constant
use wherever organic chemistry is studied; by the same year Dumas had given
the first warning of the attack which he was about to make on the great
structure of dualism raised by Berzelius. In a paper, "On Some Points of
the Atomic Theory," published in 1826, Dumas adopted the distinction made
by Avogadro between molecules and atoms, or between the small particles of
substances which remain undivided during physical actions, and the
particles, smaller than these, which are undivided during chemical actions.
But, unfortunately, Dumas did not mark these two conceptions by names
sufficiently definite to enable the readers of his memoir to bear the
distinction clearly in mind. The terms "atom" and "molecule" were not
introduced into chemistry with the precise meanings now attached to them
until some time after 1826.
Although the idea of two orders of small particles underlies all the
experimental work described by Dumas in this paper, yet the numbers which
he obtained as representing the actual atomic weights of several
elements--_e.g._ phosphorus, arsenic, tin, silicon--show that he had not
himself carried out Avogadro's hypothesis to its legitimate conclusions.
Two years after this Dumas employed the reaction wherein two volumes of
gaseous hydrochloric acid are produced by the union of one volume of
hydrogen with one volume of chlorine, as an argument which obliged him to
conclude that, if Avogadro's physical hypothesis be accepted, the molecules
of hydrogen and chlorine split, each into two parts, when these gases
combine chemically. But Dumas did not at this time conclude that the
molecular weight of hydrogen must be taken as twice its atomic weight, and
that--hydrogen being the standard substance--the molecular weights of all
gases must be represented by the specific gravities of these gases,
referred to hydrogen as 2.
I have already shortly discussed the method for finding the relative
weights of elementary atoms which is founded on Avogadro's hypothesis,
and, I think, have shown that this hypothesis leads to the definition of
"atom" as the smallest amount of an element in one molecule of any compound
of that element (see p. 142).
This deduction from Avogadro's law is now a part and parcel of our general
chemical knowledge. We wonder why it was not made by Dumas; but we must
remember that a great mass of facts has been accumulated since 1826, and
that this definition of "atom" has been gradually forced on chemists by the
cumulative evidence of those facts.
One thing Dumas did do, for which the thanks of every chemist ought to be
given him; he saw the need of a convenient method for determining the
densities of compounds in the gaseous state, and he supplied this need by
that simple, elegant and trustworthy method, still in constant use, known
as _Dumas's vapour density process_.
While Dumas was working out the details of this analytical method, which
was destined to be so powerful an instrument of research, Liebig was
engaged in similar work; he was perfecting that process for the analysis of
organic compounds which has since played so important a part in the
advancement of this branch of chemical science. The processes in use during
the first quarter of this century for determining the amounts of carbon,
hydrogen, and oxygen in compounds of those elements, were difficult to
conduct and gave untrustworthy results. Liebig adopted the principle of
the method used by Lavoisier, viz. that the carbon in a compound can be
oxidized, or burnt, to carbonic acid, and the hydrogen to water. He
contrived a very simple apparatus wherein this burning might be effected
and the products of the burning--carbonic acid and water--might be arrested
and weighed. Liebig's apparatus remains now essentially as it was presented
to the chemical world in 1830. Various improvements in details have been
made; the introduction of gas in place of charcoal as a laboratory fuel has
given the chemist a great command over the process of combustion, but in
every part of the apparatus to-day made use of in the laboratory is to be
traced the impress of the master's hand. A weighed quantity of the
substance to be analyzed is heated with oxide of copper in a tube of hard
glass; the carbon is burnt to carbonic acid and the hydrogen to water at
the expense of the oxygen of the copper oxide. Attached to the combustion
tube is a weighed tube containing chloride of calcium, a substance which
greedily combines with water, and this tube is succeeded by a set of three
or more small bulbs, blown in one piece of glass, and containing an aqueous
solution of caustic potash, a substance with which carbonic acid readily
enters into combination. The chloride of calcium tube and the potash bulbs
are weighed before and after the experiment; the increase in weight of the
former represents the amount of water, and the increase in weight of the
latter the amount of carbonic acid obtained by burning a given weight of
the compound under examination. As the composition of carbonic acid and of
water is known, the amounts of carbon and of hydrogen in one hundred parts
of the compound are easily found; the difference between the sum of these
and one hundred represents the amount of oxygen in one hundred parts of the
compound. If the compound should contain elements other than these three,
those other elements are determined by special processes, the oxygen being
always found by difference.
Soon after his settlement at Giessen Liebig turned his attention to a class
of organic compounds known as the _cyanates_; but Woehler--who, while Liebig
was in Paris in the laboratory of Gay-Lussac, was engaged in studying the
intricacies of mineral chemistry under the guidance of Berzelius--had
already entered on this field of research. The two young chemists compared
notes, recognized each other's powers, and became friends; this friendship
strengthened as life advanced, and some of the most important papers which
enriched chemical science during the next thirty years bore the joint
signatures of Liebig and Woehler.
I have already mentioned that when it was found necessary to abandon the
Lavoisierian definition of organic chemistry as the chemistry of compounds
containing carbon, hydrogen and oxygen, and sometimes also phosphorus or
nitrogen, a definition was attempted to be based on the supposed fact that
the formation of the compounds obtained from animals and plants could be
accomplished only by the agency of a living organism. But the discovery
made in 1828 by Woehler, that _urea_--a substance specially characterized by
its production in the animal economy, and in that economy only--could be
built up from mineral materials, rendered this definition of organic
chemistry impossible, and broke down the artificial barrier whereby
naturalists attempted to separate two fields of study between which Nature
made no division.
We have here another illustration of the truth of the conception which
underlies so many of the recent advances of science, which is the central
thought of the noble structure reared by the greatest naturalist of our
time, and which is expressed by one of the profoundest students of Nature
that this age has seen in the words I have already quoted from the preface
to the "Lyrical Ballads," "In Nature everything is distinct, but nothing
defined into absolute independent singleness."
From this time the progress of organic chemistry became rapid. Dumas
continued the researches upon ethers which he had commenced at Geneva, and
by the year 1829 or so he had established the relations which exist between
ethers and alcohols on the one hand, and ethers and acids on the other.
This research, a description of the details of which I cannot introduce
here as it would involve the use of many technical terms and assume the
possession by the reader of much technical knowledge, was followed by
others, whereby Dumas established the existence of a series of compounds
all possessed of the chemical properties of alcohol, all containing carbon,
hydrogen and oxygen, but differing from one another by a constant amount of
carbon and hydrogen. This discovery of a series of alcohols, distinguished
by the possession of certain definite properties whereby they were marked
off from all other so-called organic compounds, was as the appearance of a
landmark to the traveller in a country where he is without a guide. The
introduction of the comparative method of study into organic chemistry--the
method, that is, which bases classification on a comparison of large groups
of compounds, and which seeks to gather together those substances which are
like and to separate those which are unlike--soon began to bear fruit. This
method suggested to the experimenter new points of view from which to
regard groups of bodies; analogies which were hidden when a few substances
only were considered, became prominent as the range of view was widened.
What the gentle Elia calls "fragments and scattered pieces of truth,"
"hints and glimpses, germs, and crude essays at a system," became
important. There was work to be done, not only by the master spirits who,
looking at things from a central position of vantage, saw the relative
importance of the various detailed facts, but also by those who could only
"beat up a little game peradventure, and leave it to knottier heads, more
robust constitutions, to run it down."
Twenty years before the time of which we are now speaking Davy had
decomposed the alkalis potash and soda; as he found these substances to be
metallic oxides, he thought it very probable that the other well-known
alkali, ammonia, would also turn out to be the oxide of a metal. By the
electrolysis of salts formed by the action of ammonia on acids, using
mercury as one of the poles of the battery, Davy obtained a strange-looking
spongy substance which he was inclined to regard as an alloy of the
metallic base of ammonia with mercury. From the results of experiments by
himself and others, Davy adopted a view of this alloy which regarded it as
containing a _compound radicle_, or group of elementary atoms which in
certain definite chemical changes behaved like a single elementary atom.
To this compound radicle he gave the name of _ammonium_.
As an aqueous solution of potash or soda was regarded as a compound of
water and oxide of potassium or sodium, so an aqueous solution of ammonia
was regarded as a compound of water and oxide of ammonium.
When the composition of this substance, ammonium, came to be more
accurately determined, it was found that it might be best represented as a
compound atom built up of one atom of nitrogen and four atoms of hydrogen.
The observed properties of many compounds obtained from ammonia, and the
analogies observed between these and similar compounds obtained from potash
and soda, could be explained by assuming in the compound atom (or better,
in the molecule) of the ammonia salt, the existence of this group of atoms,
acting as one atom, called ammonium.
The reader will not fail to observe how essentially atomic is this
conception of compound radicle. The ultimate particle, the molecule, of a
compound has now come to be regarded as a structure built up of parts
called atoms, just as a house is a structure built up of parts called
stones and bricks, mortar and wood, etc. But there may be a closer
relationship between some of the atoms in this molecule than between the
other atoms. It may be possible to remove a group of atoms, and put another
group--or perhaps another single atom--in the place of the group removed,
without causing the whole atomic structure to fall to pieces; just as it
may be possible to remove some of the bricks from the wall of a house, or a
large wooden beam from beneath the lintels, and replace these by other
bricks or by a single stone, or replace the large wooden beam by a smaller
iron one, without involving the downfall of the entire house. The group of
atoms thus removable--the compound radicle--may exist in a series of
compounds. As we have an oxide, a sulphide, a chloride, a nitrate, etc.,
of sodium, so we may have an oxide, a sulphide, a chloride, a nitrate,
etc., of ammonium. The compounds of sodium are possessed of many properties
in common; this is partly explained by saying that they all contain one or
more atoms of the element sodium. The compounds of ammonium possess many
properties in common, and this is partly explained if we assume that they
all contain one or more atoms of the compound radicle ammonium.
The conception of compound radicle was carried by Berzelius to its utmost
limits. We have learned that the Swedish chemist regarded every molecule as
composed of two parts; in very many cases each of these parts was itself
made up of more than one kind of atom--it was a compound radicle. But the
Berzelian system tended to become too artificial: it drifted further and
further away from facts. Of the two parts composing the dual molecular
structure, one was of necessity positively, and the other negatively
electrified. The greater number of the so-called organic compounds
contained oxygen; oxygen was the most electro-negative element known; hence
most organic compounds were regarded as formed by the coming together of
one, two, or more atoms of oxygen, forming the negative part of the
molecule, with one, two, or more atoms of a compound radicle, which formed
the positive part of the molecule.
From this dualistic view of the molecule there naturally arose a
disposition to regard the compound radicles of organic chemistry as the
non-oxygenated parts of the molecules of organic compounds. An organic
compound came gradually to be regarded as a compound of oxygen with some
other elements, which were all lumped together under the name of a compound
radicle, and organic chemistry was for a time defined as the chemistry of
compound radicles.
From what has been said on p. 268, I think it will be evident that the idea
of _substitution_ is a necessary part of the original conception of
compound radicle; a group of atoms in a molecule may, it is said, be
removed, and another group, or another atom, _substituted_ for that which
is removed. Berzelius adopted this idea, but he made it too rigid; he
taught that an electro-negative atom, or compound radicle, could be
replaced or substituted only by another electro-negative atom or group of
atoms, and a positively electrified atom or group of atoms, only by another
electro-positive atom or compound radicle. Thus oxygen could perhaps be
replaced by chlorine, but certainly not by hydrogen; while hydrogen might
be replaced by a positively electrified atom, but certainly not by
chlorine.
The conceptions of compound radicles and of substitution held some such
position in organic chemistry as that which I have now attempted to
indicate when Dumas and Liebig began their work in this field.
The visitors at one of the royal _soirees_ at the Tuileries were much
annoyed by the irritating vapours which came from the wax candles used to
illuminate the apartments; Dumas was asked to examine the candles and find
the reason of their peculiar behaviour. He found that the manufacturer had
used chlorine to bleach the wax, that some of this chlorine remained in the
candles, and that the irritating vapours which had annoyed the guests of
Charles X. contained hydrochloric acid, produced by the union of chlorine
with part of the hydrogen of the wax. Candles bleached by some other means
than chlorine were in future used in the royal palaces; and the unitary
theory, which was to overthrow the dualism of Berzelius, began to arise in
the mind of Dumas.
The retention of a large quantity of chlorine by wax could scarcely be
explained by assuming that the chlorine was present only as a mechanically
held impurity. Dumas thoroughly investigated the action of chlorine on wax
and other organic compounds; and in 1834 he announced that hydrogen in
organic compounds can be exchanged for chlorine, every volume of hydrogen
given up by the original compound being replaced by an equal volume of
chlorine.
Liebig and Woehler made use of a similar conception to explain the results
which they had obtained about this time in their study of the oil of bitter
almonds, a study which will be referred to immediately.
The progress of this bold innovation made by Dumas was much advanced by the
experiments and reasonings of two French chemists, whose names ought
always to be reverenced by students of chemistry as the names of a pair of
brilliant naturalists to whom modern chemistry owes much. _Gerhardt_ was
distinguished by clearness of vision and expression; _Laurent_ by
originality, breadth of mind and power of speculation.
Laurent appears to have been the first who made a clear statement of the
fundamental conception of the unitary theory: "Many organic compounds, when
treated with chlorine lose a certain number of equivalents of hydrogen,
which passes off as hydrochloric acid. An equal number of equivalents of
chlorine takes the place of the hydrogen so eliminated; thus the physical
and chemical properties of the original substance are not profoundly
changed. The chlorine occupies the place left vacant by the hydrogen; the
chlorine plays in the new compound the same part as was played by the
hydrogen in the original compound."
The replacement of electro-positive hydrogen by electro-negative chlorine
was against every canon of the dualistic chemistry; and to say that the
physical and chemical properties of the original compound were not
profoundly modified by this replacement, seemed to be to call in question
the validity of the whole structure raised by the labours during a quarter
of a century of one universally admitted to be among the foremost chemists
of his age.
But facts accumulated. By the action of chlorine on alcohol Liebig
obtained _chloroform_ and _chloral_, substances which have since been so
largely applied to the alleviation of human suffering; but it was Dumas who
correctly determined the composition of these two compounds, and showed how
they are related to alcohol and to one another.
Liebig's reception of the corrections made by Dumas in his work furnishes a
striking example of the true scientific spirit. "As an excellent
illustration," said Liebig, "of the mode in which errors should be
corrected, the investigation of chloral by Dumas may fitly be introduced.
It carried conviction to myself, as I think to everybody else, not by the
copious number of analytical data opposed to the not less numerous results
which I had published, but because these data gave a simpler explanation
both of the formation and of the changes of the substances in question."
One of the most important contributions to the new views was made by Dumas
in his paper on the action of chlorine on acetic acid (1833), wherein he
proved that the product of this action, viz. _trichloracetic acid_, is
related to the parent substance by containing three atoms of chlorine in
place of three atoms of hydrogen in the molecule; that the new substance
is, like the parent substance, a monobasic acid; that its salts are very
analogous in properties to the salts of acetic acid; that the action of the
same reagents on the two substances is similar; and finally, that the
existence of many derivatives of these compounds could be foretold by the
help of the new hypothesis, which derivatives ought not to exist according
to the dualistic theory, but which, unfortunately for that theory, were
prepared and analyzed by Dumas.
I have alluded to a research by Liebig and Woehler on oil of bitter almonds
as marking an important stage in the advance of the anti-dualistic views.
The paper alluded to was published in 1832. At that time it was known that
_benzoic acid_ is formed by exposure of bitter-almond oil to the air.
Liebig and Woehler made many analyses of these two substances, and many
experiments on the mutual relations of their properties, whereby they were
led to regard the molecules of the oil as built up each of an atom of
hydrogen and an atom of a compound radicle--itself a compound of carbon,
hydrogen and oxygen--to which they gave the name of _benzoyl_.[13] Benzoic
acid they regarded as a compound of the same radicle with another radicle,
consisting of equal numbers of oxygen and hydrogen atoms. By the action of
chlorine and other reagents on bitter-almond oil these chemists obtained
substances which were carefully analyzed and studied, and the properties of
which they showed could be simply explained by regarding them all as
compounds of the radicle _benzoyl_ with chlorine and other atoms or groups
of atoms. But this view, if adopted, necessitated the belief that chlorine
atoms could replace oxygen atoms; and, generally, that the substitution of
an electro-positive by a negative atom or group of atoms did not
necessarily cause any great alteration in the properties of the molecule.
Thus it was that the rigid conceptions of dualism were shown to be too
rigid; that the possibility of an electro-positive radicle, or atom,
replacing another of opposite electricity was recognized; and thus the view
which regarded a compound molecule as one structure--atoms in which might
be replaced by other atoms irrespective of the mutual electrical relations
of these atoms--began to gain ground.
From this time the molecule of a compound has been generally regarded as a
unitary structure, as one whole, and the properties of the molecule as
determined by the nature, number, and arrangement of all the atoms which
together compose it.
The unitary conception of a compound molecule appeared at first to be
altogether opposed to the system of Berzelius; but as time went on, and as
fresh facts came to be known, it was seen that the new view conserved at
least one, and that perhaps the most important, of the thoughts which
formed the basis of the Berzelian classification.
Underlying the dualism of Berzelius was the conception of the molecule as
an atomic structure; this was retained in the unitary system of Dumas,
Gerhardt and Laurent.
Berzelius had insisted that every molecule is a dual structure. This is
taking too narrow a view of the possibilities of Nature, said the upholders
of the new school. _This_ molecule may have a dual structure; _that_ may be
built up of three parts. The structure of this molecule or of that can be
determined only by a careful study of its relations with other molecules.
For a time it seemed also as if the new chemistry could do without the
compound radicle which had been so much used by Berzelius; but the pressure
of facts soon drove the unitary chemists to recognize the value of that
hypothesis which looked on parts of the molecule as sometimes more closely
associated than other parts--which recognized the existence of atomic
structures within the larger molecular structures. As a house is not simply
a putting together of so many bricks, so much mortar, so many doors and
windows, so many leaden pipes, etc., but rather a definite structure
composed of parts, many of which are themselves also definite structures,
such as the window and its accessory parts, the door with its lintel and
handle, etc., so to the unitary chemists did the molecule appear to be
built up of parts, some of which, themselves composed of yet smaller parts,
discharged a particular function in the molecular economy.
A general division of a plant might describe it as a structure consisting
of a stem, a root, and leaves. Each of the parts, directly by its
individual action and indirectly by the mutual action between it and all
the other parts, contributes to the growth of the whole plant; but if the
stem, or root, or leaves be further analyzed, each is found to consist of
many parts, of fibres and cells and tissue, etc. We may liken the plant to
the molecule of an organic compound; the root, the stem and the leaves to
the compound radicles of which this molecule is built up, and the tissue,
fibres, etc., to the elementary atoms which compose these compound
radicles. The molecule is one whole, possessed of definite structure and
performing a definite function by virtue of the nature and the arrangement
of its parts.
Many years elapsed after the publication of the researches of Dumas, and of
Liebig and Woehler, before such a conception of the molecule as this was
widely accepted by chemists. The opposition of the older school, headed by
their doughty champion Berzelius, had to be overcome; the infallibility of
some of the younger members of the new school had to be checked; facts had
to be accumulated, difficulties explained, weak analogies abandoned and
strong ones rendered stronger by research; special views of the structure
of this or that molecule, deduced from a single investigation, had to be
supplemented and modified by wider views gained by the researches of many
workers. It was not till 1867 that Liebig, when asked by Dumas at a dinner
given during the French Exhibition to the foreign chemists, why he had
abandoned organic chemistry, replied that "now, with the theory of
substitution as a foundation, the edifice may be built up by workmen:
masters are no longer needed."
Laurent and Gerhardt did noble work in advancing the unitary theory; to
them is largely due the fruitful conception of types, an outcome of Dumas's
work, which owed its origin to the flickering of the wax candles in the
Tuileries during the royal _soiree_.
Chlorine can be substituted for hydrogen in acetic acid, and the product is
closely related in its properties to the parent substance; various atoms or
groups of atoms can be substituted by other groups in the derivatives of
oil of bitter almonds, but a close analogy in properties runs through all
these compounds: these facts might be more shortly expressed by saying that
acetic and trichloracetic acids belong to the same _type_, and that the
derivatives of bitter-almond oil likewise belong to one _type_.
Laurent carried this conception into inorganic chemistry. Water and potash
did not seem to have much in common, but Laurent said potash is not a
compound of oxide of potassium and water, it is rather a derivative of
water. The molecule of potash is derived from that of water by replacing
one atom of hydrogen in the latter by one atom of potassium; water and
potash belong to the same type.
Thus there was constituted _the water type_.
Light was at once thrown on many facts in organic chemistry. The analogies
between alcohol and water, some of which were first pointed out by Graham
(see p. 235), seemed to follow as a necessary consequence when the molecule
of alcohol was regarded as built on the water type. In place of two atoms
of hydrogen combined with one of oxygen, there was in the alcohol molecule
one atom of the compound radicle _ethyl_ (itself composed of carbon and
hydrogen), one atom of oxygen and one of hydrogen. Alcohol was water with
one hydrogen atom substituted by one ethyl atom; the hydrogen atom was the
atom of what we call an element, the ethyl was the atom of what we call a
compound radicle.
Gerhardt sought to refer all organic compounds to one or other of three
types--the water type, the hydrochloric acid type, and the ammonia type. As
new compounds were prepared and examined, other types had to be introduced.
To follow the history of this conception would lead us into too many
details; suffice it to say that the theory of types was gradually merged in
the wider theory of equivalency, about which I shall have a little to say
in the next chapter.
One result of the introduction of types into chemical science, associated
as it was with the unitary view of compound radicles, was to overthrow that
definition of organic chemistry which had for some time prevailed, and
which stated that organic chemistry is "the chemistry of compound
radicles." Compound radicles, it is true, were more used in explaining the
composition and properties of substances obtained from animals and
vegetables than of mineral substances, but a definition of one branch of a
science which practically included the other branch, from which the first
was to be defined, could not be retained. Chemists became gradually
convinced that a definition of organic chemistry was not required; that
there was no distinction between so-called organic and inorganic compounds;
and they have consented, but I scarcely think will much longer consent, to
retain the terms "organic" and "inorganic," only because these terms have
been so long in use. The known compounds of the element carbon are so
numerous, and they have been so much studied and so well classified, that
it has become more convenient for the student of chemistry to consider them
as a group, to a great extent apart from the compounds of the other
elements; to this group he still often gives the name of "organic
compounds."
* * * * *
Liebig continued to hold the chair of Chemistry in the University of
Giessen until the year 1852, when he was induced by the King of Bavaria to
accept the professorship of the same science in the University of Munich.
During the second quarter of this century Giessen was much resorted to by
students of chemistry from all parts of the world, more especially from
England. Many men who afterwards made their mark in chemical discovery
worked under the guidance of the professor of Stockholm, but Giessen has
the honour of being the place where a well-appointed chemical laboratory
for scientific research was first started as a distinctly educational
institution. The fame of Liebig as a discoverer and as a teacher soon
filled the new institution with students, who were stirred to enthusiasm as
they listened to his lectures, or saw him at work in his laboratory.
"Liebig was not exactly what is called a fluent speaker," says Professor
Hofmann, of Berlin, "but there was an earnestness, an enthusiasm in all he
said, which irresistibly carried away the hearer. Nor was it so much the
actual knowledge he imparted which produced this effect, as the wonderful
manner in which he called forth the reflective powers of even the least
gifted of his pupils. And what a boon was it, after having been stifled by
an oppressive load of facts, to drink the pure breath of science such as it
flowed from Liebig's lips! what a delight, after having perhaps received
from others a sack full of dry leaves, suddenly in Liebig's lectures to see
the living, growing tree!... We felt then, we feel still, and never while
we live shall we forget, Liebig's marvellous influence over us; and if
anything could be more astonishing than the amount of work he did with his
own hands, it was probably the mountain of chemical toil which he got us to
go through. Each word of his carried instruction, every intonation of his
voice bespoke regard; his approval was a mark of honour, and of whatever
else we might be proud, our greatest pride of all was having him for our
master.... Of our young winnings in the noble playground of philosophical
honour, more than half were free gifts to us from Liebig, and to his
generous nature no triumphs of his own brought more sincere delight than
that which he took in seeing his pupils' success, and in assisting, while
he watched, their upward struggle."
Liebig had many friends in England. He frequently visited this country, and
was present at several meetings of the British Association. At the meeting
of 1837 he was asked to draw up a report on the progress of organic
chemistry; he complied, and in 1840 presented the world with a book which
marks a distinct epoch in the applications of science to industrial
pursuits--"Chemistry in its Applications to Agriculture and Physiology."
In this book, and in his subsequent researches and works,[14] Liebig
established and enforced the necessity which exists for returning to the
soil the nourishing materials which are taken from it by the growth of
crops; he suggested that manure rich in the salts which are needed by
plants might be artificially manufactured, and by doing this he laid the
foundation of a vast industry which has arisen during the last two decades.
He strongly and successfully attacked the conception which prevailed among
most students of physiology at that time, that chemical and physical
generalizations could not be applied to explain the phenomena presented by
the growth of living organisms. He was among the first to establish, as an
induction from the results of many and varied experiments, the canon which
has since guided all teachers of the science of life, that a true knowledge
of biology must be based on a knowledge of chemistry and physics.
But Liebig was not content to establish broad generalizations and to leave
the working out of them to others; he descended from the heights of
philosophical inquiry, and taught the housewife to make soup wherein the
greatest amount of nourishment was conveyed to the invalid in the most
easily digestible form; and has he not, by bringing within the reach of
every one a portion of the animal nourishment which else had run to waste
in the pampas of South America or the sheep-runs of Australia, made his
name, in every English home, familiar as a household word?
On the death of Berzelius in 1848, it was to Liebig that every chemist
looked for a continuation of the annual Report on the progress of
chemistry, which had now become the central magazine of facts, whither each
worker in the science could resort to make himself acquainted with what had
been done by others on any subject which he proposed to investigate. From
that time to the present day Liebig's _Annalen_ has been the leading
chemical journal of the world.
Of the other literary work of Liebig--of his essays, his celebrated
"Chemical Letters," his many reports, his severe and sometimes harsh
criticisms of the work of others--of the details of the three hundred
original papers wherein he embodied the results of his researches, I have
not time, nor would this be the place, to speak.
Honoured by every scientific society of any note in the world, crowned with
the highest reward which England and France can offer to the man of science
who is not an Englishman or a Frenchman--the Copley Medal and the
associateship of the Institute--honoured and respected by every student of
science, loved by each of the band of ardent natures whom he had trained
and sent forth to battle for the good of their race, and, best of all,
working himself to the last in explaining the wonders of Nature, he "passed
into the silent land" on the 18th of April 1873, leaving the memory of a
life nobly devoted to the service of humanity, and the imperishable record
of many truths added to the common stock of the race.
* * * * *
The life-work of Dumas, other than that which I have already sketched, is
so manifold and so varied, that to do more than refer to one or two leading
points would carry us far beyond the limits within which I have tried to
keep throughout this book. In one of his earliest papers Dumas adopted the
atomic theory as the corner-stone of his chemical system; he was thus led
to an experimental revision of the values generally accepted for the
atomic weights of some of the elements. Among these revisions, that of the
atomic weight of carbon holds a most important place, partly because of the
excellency of the work, but more because of the other inquiries to which
this work gave rise.
Dumas's experiments were summed up in the statement that the atom of carbon
is twelve times heavier than the atom of hydrogen. The experimental methods
and the calculations used in this determination involved a knowledge of the
atomic weight of oxygen; in order accurately to determine the value to be
assigned to this constant, Dumas, in conjunction with Boussingault,
undertook a series of experiments on the synthesis of water, which forms
one of the classical researches of chemistry, and wherein the number 16 was
established as representing the atomic weight of oxygen. Stas, from
experiments conducted at a later time with the utmost care and under
conditions eminently fitted to gain accurate results, obtained the number
15.96, in place of 16, for the atomic weight of oxygen; but in a paper
recently published by the veteran Dumas, a source of error is pointed out
which Stas had overlooked in his experiments, and it is shown that this
error would tend slightly to increase the number obtained by Stas.
As the values assigned to the atomic weights of the elements are the very
fundamental data of chemistry, and as we are every day more clearly
perceiving that the mutual relations between the properties of elements and
compounds are closely connected with the relative weights of the elementary
atoms, we can scarcely lay too much stress on such work as this done by
Dumas and Stas. Not many years after the publication of Dalton's "New
System," the hypothesis was suggested by Prout that the atomic weights of
all the elements are represented by whole numbers--that of hydrogen being
taken as unity--that the atom of each element is probably formed by the
putting together of two, three, four, or more atoms of hydrogen, and that
consequently there exists but a single elementary form of matter. Among the
upholders of this hypothesis Dumas has held an important place. He modified
the original statement of Prout, and suggested that all atomic weights are
whole multiples of half of that of hydrogen (that is, are whole multiples
of 1/2). The experiments of Stas seemed to negative this view, but later
work--more especially the important critical revision of the results
obtained by all the most trustworthy workers, conducted by Professor Clarke
of Cincinnati, and published by the Smithsonian Institution as part of
their series of "Constants of Nature"--has shown that we are in no wise
warranted by facts in rejecting Prout's hypothesis as modified by Dumas,
but that the balance of evidence is at present rather in its favour.
It would be altogether out of place to discuss here an hypothesis which
leads to some of the most abstruse speculations as to the nature of matter
in which chemists have as yet ventured to indulge. I mention it only
because it illustrates the far-reaching nature of the researches of the
chemist whose work we are now considering, and also because it shows the
shallowness of the scoffs in which some partly educated people indulge when
they see scientific men occupying themselves for years with attempts to
solve such a minute and, as they say, trivial question as whether the
number 15.96 or the number 16 is to be preferred as representing the atomic
weight of oxygen; "for in every speck of dust that falls lie hid the laws
of the universe, and there is not an hour that passes in which you do not
hold the infinite in your hand."
Another and very different subject, which has been placed on a firm basis
by the researches of Dumas, is the chemistry of fermentation. By his work
on the action of beer-yeast on saccharine liquids, Dumas proved Liebig's
view to be untenable--according to which the conversion of sugar into
alcohol is brought about by the influence of chemical changes proceeding in
the ferment; also that the view of Berzelius, who regarded alcoholic
fermentation as due simply to the contact of the ferment with the sugar,
was opposed to many facts; and lastly, Dumas showed that the facts were
best explained by the view which regarded the change of sugar into alcohol
as in no way different from other purely chemical changes, but as a change
brought about, so far as our present knowledge goes, only by the agency of
a growing organism of low form, such as yeast.
In 1832 Dumas established at his own expense a laboratory for chemical
research. When the Revolution of 1848 broke out Dumas's means were much
diminished, and he could no longer afford to maintain his laboratory. The
closing of this place, where so much sound work had been done, was
generally regarded as a calamity to science. About this time Dumas received
a visit from a person of unprepossessing appearance, who accosted him thus:
"They assert that you have shut up your laboratory, but you have no right
to do so. If you are in need of money, there," throwing a roll of
bank-notes on the table, "take what you want. Do not stint yourself; I am
rich, a bachelor, and have but a short time to live." Dumas's visitor
turned out to be Dr. Jecker. He assured Dumas that he was now only paying a
debt, since he had made a fortune by what he had learnt in the medical
schools of Paris. Dumas could not however in those troublous times turn his
mind continuously to experimental research, and therefore declined Dr.
Jecker's offer with many protestations of good will and esteem.
New work now began to press upon Dumas; his energy and his administrative
powers were demanded by the State. Elected a member of the National
Assembly in 1848, he was soon called by the President of the Republic to
office as Minister of Agriculture and Commerce. He was made a senator under
the second empire. He entered the municipal council of Paris about 1854,
and was soon elected to the presidency. Under his presidency the great
scheme for providing Paris with spring-water carried by aqueducts and
tunnels was successfully accomplished; many improvements were made in the
drainage of the city; the cost of gas was decreased, while the quality was
improved, the constancy of the supply insured, and the appliances for
burning the gas in the streets were altered and rendered more effective.
Nominated to succeed Pelouze as Master of the Mint in 1868, Dumas held this
honourable and important position only until the Franco-German war of 1870.
Since that date he has relinquished political life; but as Permanent
Secretary of the Academy Dumas now fills the foremost place in all affairs
connected with science, whether pure or applied, in the French capital.
In the work of these two chemists, Liebig and Dumas, we find admirable
illustrations of the scientific method of examining natural appearances.
In the broad general views which they both take of the phenomena to be
studied, and the patient and persevering working out of details, we have
shown us the combination of powers which are generally found in separate
individuals.
Dumas has always insisted on the need of comparing properties and reactions
of groups of bodies, before any just knowledge can be gained as to the
position of a single substance in the series studied by the chemist. It has
been his aim as a teacher, we are assured by his friend, Professor Hofmann,
never to present to his students "an isolated phenomenon, or a notion not
logically linked with others." To him each chemical compound is one in a
series which connects it directly with many other similar compounds, and
indirectly with other more or less dissimilar compounds.
Amid the overwhelming mass of facts which threaten nowadays to bury the
science of chemistry, and crush the life out of it by their weight, Dumas
tracks his way by the aid of general principles; but these principles are
themselves generalized from the facts, and are not the offspring of his own
fancy.
We have, I think, found that throughout the progress of chemical science
two dangers have beset the student. He has been often tempted to accumulate
facts, to amass analytical details, to forget that he is a chemist in his
desire to perfect the instrument of analysis by the use of which he raises
the scaffolding of his science; on the other hand, he has been sometimes
allured from the path of experiment by his own day-dreams. The discoveries
of science have been so wonderful, and the conceptions of some of those who
have successfully prosecuted science have been so grand, that the student
has not unfrequently been tempted to rest in the prevailing theories of the
day, and, forgetting that these ought only "to afford peaceful lodgings to
the intellect for the time," he has rather allowed them to circumscribe it,
until at last the mind "finds difficulty in breaking down the walls of what
has become its prison, instead of its home."
We may think that Dumas fell perhaps slightly into the former of these
errors, when he did not allow his imagination a little more scope in
dealing with the conception of "atom" and "molecule," the difference
between which he had apprehended but not sufficiently marked by the year
1826 (see p. 261).
We know, from his own testimony, that Liebig once fell into the latter
error and that the consequences were disastrous. "I know a
chemist"--meaning himself--"who ... undertook an investigation of the
liquor from the salt-works. He found iodine in it, and observed, moreover,
that the iodide of starch turned a fiery yellow by standing over-night. The
phenomenon struck him; he saturated a large quantity of the liquor with
chlorine, and obtained from this, by distillation, a considerable quantity
of a liquid which coloured starch yellow, and externally resembled chloride
of iodine, but differed from this compound in many properties. He
explained, however, every discrepancy with satisfaction to himself; he
contrived for himself a theory. Several months later, he received a paper
of M. Balard's," announcing the discovery of bromine, "and on that same day
he was able to publish the results of experiments on the behaviour of
bromine with iron, platinum, and carbon; for Balard's bromine stood in his
laboratory, labelled _liquid chloride of iodine_. Since that time he makes
no more theories unless they are supported and confirmed by trustworthy
experiments; and I can positively assert that he has not fared badly by so
doing."
Another point which we notice in the life-work of these two chemists is
their untiring labour. They were always at work; wherever they might be,
they were ready to notice passing events or natural phenomena, and to draw
suggestions from these. As Davy proved the elementary character of iodine
and established many of the properties of this substance during a visit to
Paris, so we find Dumas making many discoveries during brief visits paid to
his friends' laboratories when on excursions away from Paris. During a
visit to Aix-les-Bains, he noticed that the walls of the bath-room were
covered with small crystals of sulphate of lime. The waters of the bath, he
knew, were charged with sulphuretted hydrogen, but they contained no
sulphuric acid, nor could that acid be detected in the air of the
bath-rooms. This observation was followed up by experiments which proved
that a porous material, such as a curtain or an ordinary plastered wall, is
able to bring about the union of oxygen with sulphuretted hydrogen,
provided moisture be present and a somewhat high temperature be
maintained.
Again, we find Liebig and Dumas characterized by great mental honesty.
"There is no harm in a man committing mistakes," said Liebig, "but great
harm indeed in his committing none, for he is sure not to have worked....
An error you have become cognizant of, do not keep in your house from night
till morning."
Students of science, more than any other men, ought to be ready to
acknowledge and correct the errors into which they fall. It is not
difficult for them to do this: they have only to be continually going to
Nature; for there they have a court of appeal always ready to hear their
case, and to give an absolutely unbiased judgment: they have but to bring
their theories and guesses to this judge to have them appraised at their
true value.
FOOTNOTES:
[13] "In reviewing once more the facts elicited by our inquiry, we find
them arranged around a common centre, a group of atoms preserving intact
its nature, amid the most varied associations with other elements. This
stability, this analogy, pervading all the phenomena, has induced us to
consider this group as a sort of compound element, and to designate it by
the special name of _benzoyl_."--Liebig and Woehler, 1832.
[14] "Animal Chemistry, or Chemistry in its Applications to Physiology and
Pathology," 1842. "Researches on the Chemistry of Food," 1847. "The Natural
Laws of Husbandry," 1862.