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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 cla.s.s 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 a.r.s.enic 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 cla.s.s 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 cla.s.sification 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 a.n.a.lysing 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 cla.s.s 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 cla.s.sical 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 pa.s.sed 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 pa.s.sed 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 b.u.t.tons, 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 pa.s.s 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 a.n.a.lysis 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, a.r.s.enic, 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 ma.s.s of facts has been acc.u.mulated since 1826, and that this definition of "atom" has been gradually forced on chemists by the c.u.mulative 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 a.n.a.lytical 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 a.n.a.lysis 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 quant.i.ty of the substance to be a.n.a.lyzed is heated with oxide of copper in a tube of hard gla.s.s; 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 gla.s.s, 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 cla.s.s of organic compounds known as the _cyanates_; but Wohler--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 Wohler.
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 Wohler, 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 ill.u.s.tration of the truth of the conception which underlies so many of the recent advances of science, which is the central thought of the n.o.ble 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 a.s.sume 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 cla.s.sification 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; a.n.a.logies 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 const.i.tutions, 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 pota.s.sium 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 a.n.a.logies observed between these and similar compounds obtained from potash and soda, could be explained by a.s.suming 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 a.s.sume 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 _subst.i.tution_ 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, _subst.i.tuted_ 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 subst.i.tuted 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 subst.i.tution 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 quant.i.ty of chlorine by wax could scarcely be explained by a.s.suming 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 Wohler 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 pa.s.ses 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 acc.u.mulated. 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 ill.u.s.tration," said Liebig, "of the mode in which errors should be corrected, the investigation of chloral by Dumas may fitly be introduced.