Part 16 (2/2)
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 friends.h.i.+p 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 relations.h.i.+p 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.
<script>