a question. No
man sees all through a discovery at once.' Thus Descartes's notion of
matter, and his explanation of motion, would be put aside as trivial
by a physiologist or a crystallographer of the present day. They are
not descriptions of the state of the question. And yet a desire
sometimes shows itself in distinguished quarters to bind us own to
conceptions which passed muster in the infancy of knowledge, but which
are wholly incompatible with our present enlightenment. Mr.
Martineau, I think, errs when he seeks to hold me to views enunciated
by 'Democritus and the mathematicians.' That definitions should change
as knowledge advances is in accordance both with sound sense and
scientific practice. When, for example, the undulatory theory was
started, it was not imagined that the vibrations of light could be
transverse to the direction of propagation. The example of sound was
at hand, which was a case of longitudinal vibration. Now the
substitution of transverse for longitudinal vibrations in the case of
light involved a radical change of conception as to the mechanical
properties of the luminiferous medium. But though this change went so
far as to fill space with a substance, possessing the properties of a
solid, rather than those of a gas, the change was accepted, because
the newly discovered facts imperatively demanded it. Following Mr.
Martineau's example, the opponent of the undulatory theory might
effectually twit the holder of it on his change of front. 'This
aether of yours,' he might say, 'alters its style with every change
of service. Starting as a beggar, with scarce a rag of 'property' to
cover its bones, it turns up as a prince when large undertakings are
wanted. You had some show of reason when, with the case of sound
before you, you assumed your aether to be a gas in the last extremity
of attenuation. But now that new service is rendered necessary by new
facts, you drop the beggar's rags, and accomplish an undertaking,
great and princely enough in all conscience; for it implies that not
only planets of enormous weight, but comets with hardly any weight at
all, fly through your hypothetical solid without perceptible loss of
motion.' This would sound very cogent, but it would be very vain.
Equally vain, in my opinion, is Mr. Martineau's contention that we are
not justified in modifying, in accordance with advancing knowledge,
our notions of matter.

Before parting from Professor Knight, le