by many eminent physicists, though of
course none have called in question the necessity for some interstellar
medium for the transmission of thermal and luminous vibrations.
This scepticism has been, I think, partially justified by the many
difficulties encompassing the conception, into which, however, we need
not here enter. That light and heat cannot be conveyed by any of the
ordinary sensible forms of matter is unquestionable. None of the forms
of sensible matter can be imagined sufficiently elastic to propagate
wave-motion at the rate of one hundred and eighty-eight thousand miles
per second. Yet a ray of light is a series of waves, and implies some
substance in which the waves occur. The substance required is one which
seems to possess strangely contradictory properties. It is commonly
regarded as an "ether" or infinitely rare substance; but, as Professor
Jevons observes, we might as well regard it as an infinitely solid
"adamant." "Sir John Herschel has calculated the amount of force which
may be supposed, according to the undulatory theory of light, to be
exerted at each point in space, and finds it to be 1,148,000,000,000
times the elastic force of ordinary air at the earth's surface, so that
the pressure of the ether upon a square inch of surface must be about
17,000,000,000,000, or seventeen billions of pounds." [4] Yet at the
same time the resistance offered by the ether to the planetary motions
is too minute to be appreciable. "All our ordinary notions," says
Professor Jevons, "must be laid aside in contemplating such an
hypothesis; yet [it is] no more than the observed phenomena of light and
heat force us to accept. We cannot deny even the strange suggestion of
Dr. Young, that there may be independent worlds, some possibly existing
in different parts of space, but others perhaps pervading each other,
unseen and unknown, in the same space. For if we are bound to admit the
conception of this adamantine firmament, it is equally easy to admit a
plurality of such."

[4] Jevons's Principles of Science, Vol. II. p. 145. The figures,
which in the English system of numeration read as seventeen billions,
would in the American system read as seventeen trillions.

The ether, therefore, is unlike any of the forms of matter which we
can weigh and measure. In some respects it resembles a fluid, in some
respects a solid. It is both hard and elastic to an almost inconceivable
degree. It fills all material bodies like a