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.
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 associated 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.