ena. Empirical
investigation begins by single perceptions, which are afterwards classed
according to their analogy or dissimilarity. Observation is succeeded at
a much later epoch by experiment, in which phenomena are made to arise
under conditions previously determined on by the experimentalist, guided
by preliminary hypotheses, or a more or less just intuition of the
connection of natural objects and forces.

The results obtained by observation and experiment lead by the path of
induction and analogy to the discovery of empirical laws, and these
successive phases in the application of human intellect have marked
different epochs in the life of nations. It has been by adhering closely
to this inductive path that the great mass of facts has been accumulated
which now forms the solid foundation of the natural sciences.

Two forms of abstraction govern the whole of this class of
knowledge--_viz._, the determination of quantitative relations,
according to number and magnitude; and relations of quality, embracing
the specific properties of heterogeneous matter.

The first of these forms, more accessible to the exercise of thought,
belongs to the domain of mathematics; the other, more difficult to
seize, and apparently more mysterious, to that of chemistry. In order to
submit phenomena to calculation, recourse is had to a hypothetical
construction of matter by a combination of molecules and atoms whose
number, form, position, and polarity determine, modify, and vary the
phenomena.

We are yet very far from the time when a reasonable hope could be
entertained of reducing all that is perceived by our senses to the unity
of a single principle; but the partial solution of the problem--the
tendency towards a general comprehension of the phenomena of the
universe--does not the less continue to be the high and enduring aim of
all natural investigation.

_III.--Distribution of Matter in Space_

A physical cosmography, or picture of the universe, should begin, not
with the earth, but with the regions of space--the distribution of
matter in the universe.

We see matter existing in space partly in the form of rotating and
revolving spheroids, differing greatly in density and magnitude, and
partly in the form of self-luminous vapour dispersed in shining nebulous
spots or patches. The nebulae present themselves to the eye in the form
of round, or nebulous discs, of small apparent magnitude, either single
or in pairs, which are