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RADIANT HEAT AND ITS RELATIONS. 215

of light require a medium for their formation and propaga-
tion, but we cannot see, or feel, or taste, or smell this
medium. How, then, has its existence been established ?
By showing that by the assumption of this wonderful
intangible ether all the phenomena of optics are accounted
for with a fulness and clearness and conclusiveness which
leave no desire of the intellect unfulfilled. When the law
of gravitation first suggested itself to the mind of Newton,
what did he do ? He set himself to examine whether it
accounted for all the facts. He determined the courses of
the planets; he calculated the rapidity of the moon’s fall
toward the earth; he considered the preceSsion of the
equinoxes, the ebb and flow of the tides, and found all
explained by the law of gravitation. He therefore regarded
this law as established, and the verdict of science sub-
sequently confirmed his conclusion. On similar, and, if
possible, on stronger grounds, we found our belief in the
existence of the universal ether. It explains facts far
more various and complicated than those on which New-
ton based his law. If a single phenOmenon could be
pointed out which the ether is proved incompetent to
explain, we should have to give it up; but no such phe-
nomenon has ever been pointed out. It is, therefore, at
least as certain that Space is filled with a medium by
means of which suns and stars diffuse their, radiant power,
as that it is traversed by that force which holds, not only
our planetary system, but the immeasurable heavens them-
selves, in its grasp.

There is no more wonderful instance than this of the
production of a line of thought from the world of the senses
into the region of pure imagination. I mean by imagination
here, not that'play of fancy which can give to “ airy nothing
a local habitation and a name,” but that power which
enables the mind to conceive realities which lie beyond the
range of the senses—to present to itself distinct physical

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