ON RADIANT HEAT. 57 
imagination here, not that play of fancy which can give 
to airy nothings 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 images of processes which, though mighty in the 
aggregate beyond all conception, are so minute individually 
as to elude all observation. It is the waves of air excited 
by a tuning-fork which render its vibrations audible. It 
is the waves of ether sent forth from those lamps overhead 
which render them luminous to us; but so minute are 
these waves, that it would take from 30,000 to 60,000 of 
them placed end to end to cover a single inch. Their 
number, however, compensates for their minuteness. Tril- 
lions of them have entered your eyes, and hit the retina at 
the backs of your eyes, in the time consumed in the utter- 
ance of the shortest sentence of this discourse. This is 
the steadfast result of modern research; but we never 
could have reached it without previous discipline. We 
never could have measured the waves of light, nor even 
imagined them to exist, had we not previously exercised 
ourselves among the waves of sound. Sound and light are 
now mutually helpful, the conceptions of each being ex- 
panded, strengthened, and defined by the conceptions of 
the other. 
The ether which conveys the pulses of light and heat 
not only fills celestial space, swathing suns, and planets, 
and moons, but it also encircles the atoms of which these 
bodies are composed. It is the motion of these atoms, and 
not that of any sensible parts of bodies, that the ether con- 
veys. This motion is the objective cause of what, in our 
sensations, are light and heat. An atom, then, sending 
its pulses through the ether, resembles a tuning-fork 
sending its pulses through the air. Let us look for a 
moment at this thrilling medium, and briefly consider its 
relation to the bodies whose vibrations it conveys. Dif- 
ferent bodies, when heated to the same temperature, pos- 
sess very different powers of agitating the ether: some 
are good radiators, others are bad radiators: which means 
that some are so constituted as to communicate their 
atomic motion freely to the ether, producing therein pow- 
erful undulations; while the atoms of others are unable 
thus to communicate their motions, but glide through 
the medium without materially disturbing its repose, 
