the polarisation of light
led to experiments which ended in the discovery of the polarisation of
heat--a discovery that could never have been made without the antecedent
one. Thus, too, the known refrangibility of light and heat lately
produced the inquiry whether sound also is not refrangible; which on
trial it turns out to be.

In some cases, indeed, it is only by the aid of conceptions derived from
one class of phenomena that hypotheses respecting other classes can be
formed. The theory, at one time favoured, that evaporation is a solution
of water in air, was an assumption that the relation between water and
air is _like_ the relation between salt and water; and could never have
been conceived if the relation between salt and water had not been
previously known. Similarly the received theory of evaporation--that it
is a diffusion of the particles of the evaporating fluid in virtue of
their atomic repulsion--could not have been entertained without a
foregoing experience of magnetic and electric repulsions. So complete in
recent days has become this _consensus_ among the sciences, caused
either by the natural entanglement of their phenomena, or by analogies
in the relations of their phenomena, that scarcely any considerable
discovery concerning one order of facts now takes place, without very
shortly leading to discoveries concerning other orders.

To produce a tolerably complete conception of this process of
scientific evolution, it would be needful to go back to the beginning,
and trace in detail the growth of classifications and nomenclatures; and
to show how, as subsidiary to science, they have acted upon it, and it
has reacted upon them. We can only now remark that, on the one hand,
classifications and nomenclatures have aided science by continually
subdividing the subject-matter of research, and giving fixity and
diffusion to the truths disclosed; and that on the other hand, they have
caught from it that increasing quantitativeness, and that progress from
considerations touching single phenomena to considerations touching the
relations among many phenomena, which we have been describing.

Of this last influence a few illustrations must be given. In chemistry
it is seen in the facts, that the dividing of matter into the four
elements was ostensibly based upon the single property of weight; that
the first truly chemical division into acid and alkaline bodies, grouped
together bodies which had not simply one pro