Heroes of Science: Chemists

by M. M. Pattison Muir

Liebig and Wöhler 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 Wöhler 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 Wöhler 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.