bsisting in this stage of growth, and afterwards. Let us look at a few
cases.

The theoretic law of the velocity of sound enunciated by Newton on
purely mechanical considerations, was found wrong by one-sixth. The
error remained unaccounted for until the time of Laplace, who,
suspecting that the heat disengaged by the compression of the undulating
strata of the air, gave additional elasticity, and so produced the
difference, made the needful calculations and found he was right. Thus
acoustics was arrested until thermology overtook and aided it. When
Boyle and Marriot had discovered the relation between the density of
gases and the pressures they are subject to; and when it thus became
possible to calculate the rate of decreasing density in the upper parts
of the atmosphere, it also became possible to make approximate tables of
the atmospheric refraction of light. Thus optics, and with it astronomy,
advanced with barology. After the discovery of atmospheric pressure had
led to the invention of the air-pump by Otto Guericke; and after it had
become known that evaporation increases in rapidity as atmospheric
pressure decreases; it became possible for Leslie, by evaporation in a
vacuum, to produce the greatest cold known; and so to extend our
knowledge of thermology by showing that there is no zero within reach of
our researches. When Fourier had determined the laws of conduction of
heat, and when the Earth's temperature had been found to increase below
the surface one degree in every forty yards, there were data for
inferring the past condition of our globe; the vast period it has taken
to cool down to its present state; and the immense age of the solar
system--a purely astronomical consideration.

Chemistry having advanced sufficiently to supply the needful materials,
and a physiological experiment having furnished the requisite hint,
there came the discovery of galvanic electricity. Galvanism reacting on
chemistry disclosed the metallic bases of the alkalies, and inaugurated
the electro-chemical theory; in the hands of Oersted and Ampere it led
to the laws of magnetic action; and by its aid Faraday has detected
significant facts relative to the constitution of light. Brewster's
discoveries respecting double refraction and dipolarisation proved the
essential truth of the classification of crystalline forms according to
the number of axes, by showing that the molecular constitution depends
upon the axes. In these and in n