It carried conviction to myself, as I think to everybody else, not by the copious number of a.n.a.lytical 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 mon.o.basic acid; that its salts are very a.n.a.logous 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 a.n.a.lyzed by Dumas.

I have alluded to a research by Liebig and Wohler 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 Wohler made many a.n.a.lyses 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 a.n.a.lyzed 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 subst.i.tution 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 cla.s.sification.

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 a.s.sociated 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 a.n.a.lyzed, 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 Wohler, 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 acc.u.mulated, difficulties explained, weak a.n.a.logies 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 subst.i.tution as a foundation, the edifice may be built up by workmen: masters are no longer needed."

Laurent and Gerhardt did n.o.ble 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 subst.i.tuted 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 subst.i.tuted by other groups in the derivatives of oil of bitter almonds, but a close a.n.a.logy 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 pota.s.sium 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 pota.s.sium; water and potash belong to the same type.

Thus there was const.i.tuted _the water type_.

Light was at once thrown on many facts in organic chemistry. The a.n.a.logies 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 subst.i.tuted 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, a.s.sociated 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 cla.s.sified, 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 inst.i.tution. The fame of Liebig as a discoverer and as a teacher soon filled the new inst.i.tution 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 n.o.ble 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 a.s.sisting, 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 a.s.sociation. 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 a.s.sociateship of the Inst.i.tute--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 "pa.s.sed into the silent land" on the 18th of April 1873, leaving the memory of a life n.o.bly 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 a.s.signed to this constant, Dumas, in conjunction with Boussingault, undertook a series of experiments on the synthesis of water, which forms one of the cla.s.sical 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 1596, 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 a.s.signed 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 Inst.i.tution 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 ill.u.s.trates 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 1596 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 pa.s.ses 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 a.s.sert 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 a.s.sured 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 a.s.sembly 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 munic.i.p.al 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 ill.u.s.trations 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 a.s.sured 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 ma.s.s 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 acc.u.mulate facts, to ama.s.s a.n.a.lytical details, to forget that he is a chemist in his desire to perfect the instrument of a.n.a.lysis 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 circ.u.mscribe 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 quant.i.ty of the liquor with chlorine, and obtained from this, by distillation, a considerable quant.i.ty 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 a.s.sert 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 pa.s.sing 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.