Now, if two elementary atoms unite, the compound atom must--according to the Berzelian views--be characterized either by positive or negative electricity. This compound atom, if positive, will exhibit affinity for other compound atoms in which negative electricity predominates; if negative, it will exhibit affinity for other positively electrified compound atoms. If two compound atoms unite chemically, the complex atom so produced will, again, be characterized by one or other of the two electricities, and as it is positive or negative, so will it exhibit affinity for positively or negatively electrified complex atoms. Thus Berzelius and his followers regarded every compound atom, however complex, as essentially built up of two parts, one of which was positively and the other negatively electrified, and which were held together chemically by virtue of the mutual attractions of these electricities; they regarded every compound atom as a _dual_ structure. The classification adopted by Berzelius was essentially a dualistic classification. His system has always been known in chemistry as _dualism_. Berzelius divided compound atoms (we should now say molecules) into three groups or orders-- _Compound atoms of the first order_, formed by the immediate combination of atoms of two, or in organic compounds of three, elementary substances. _Compound atoms of the second order_, formed by the combination of atoms of an element with atoms of the first order, or by the combination of two or more atoms of the first order. _Compound atoms of the third order_, formed by combination of two or more atoms of the second order. When an atom of the third order was decomposed by an electric current, it split up, according to the Berzelian teaching, into atoms of the second order--some positively, others negatively electrified. When an atom of the second order was submitted to electrolysis, it decomposed into atoms of the first order--some positively, others negatively electrified. Berzelius said that a base is an electro-positive oxide, and an acid is an electro-negative oxide. The more markedly positive an oxide is, the more basic it is; the more negative it is, the more is it characterized by acid properties. One outcome of this teaching regarding acids and bases was to overthrow the Lavoisierian conception of oxygen as the acidifying element. Some oxides are positive, others negative, said Berzelius; but acids are characterized by negative electricity, therefore the presence of oxygen in a compound does not always confer on that compound acid properties.