Any pair of the general elements, the interstices of one of which are
the only interstices for receiving and retaining the atoms of the
other, or that can be occupied by the atoms of any other of the general
elements, such elements are correlatives.
Elementary co-relation is conspicuous in the opposite polarities
of the loadstone, magnet, and crystals, and all bodies subject to
polarization, which includes the animal frame. Similar co-relation is
evinced between the galvanic fluids, those of the pile, and those named
electricity; likewise between oxygen and hydrogen, the oxygenating
element and nitrogen, acids and alkalies and all mutually neutralizing
substances. Still it is not meant that all the general elements are so
paired; doubtless, there are several ratios of size between the atoms
of the different elements, for the purpose of multiplying variety among
formations, the substance of which is of the same species throughout.
Possibly the correlative principle gave rise to the ideal scale of
_chymical affinities_, subsequently refined to _affections of matter_.
Naturally, correlative elements will be found together, as are nitrogen
and the imponderable element; also the magnetic fluids common to iron.