Part 17 (1/2)
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 professors.h.i.+p 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 astonis.h.i.+ng 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 nouris.h.i.+ng 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.