In the observation of a phenomenon which at first sight appears to
be wholly isolated, how often may be concealed the germ of a great
discovery! Thus, when Galvani first stimulated the nervous fibre of
the frog by the accidental contact of two heterogeneous metals, his
contemporaries could never have anticipated that the action of the
voltaic pile would discover to us in the alkalies metals of a silver
lustre, so light as to swim on water, and eminently inflammable; or
that it would become a powerful instrument of chemical analysis, and at
the same time a thermoscope and a magnet. When Huyghens first observed,
in 1678, the phenomenon of the polarisation of light, exhibited in the
difference between two rays into which a pencil of light divides itself
in passing through a doubly refracting crystal, it could not have been
foreseen that a century and a half later the great philosopher Arago
would, by his discovery of _chromatic polarisation_, be led to discern,
by means of a small fragment of Iceland spar, whether solar light
emanates from a solid body or a gaseous covering; or whether comets
transmit light directly, or merely by reflection.--_Humboldt’s Cosmos_,
vol. i.