I have alluded to a research by Liebig and Woehler 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 Woehler 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. 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 classification. 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.