ine units of heat, while the other emits seventy-four.
This, surely, is a great difference. Here, however, is a second cube,
having two of its surfaces coated with the same powders, the only
difference being that the powders are laid on by means of a
transparent gum. Both surfaces are now absolutely alike in radiative
power. Both of them emit somewhat more than was emitted by either of
the unvarnished powders, simply because the gum employed is a better
radiator than either of them. Excluding all varnish, and comparing
white with white, vast differences are found; comparing black with
black, they are also different; and when black and white are compared,
in some cases the black radiates far more than the white, while in
other cases the white radiates far more than the black. Determining,
moreover, the absorptive power of those powders, it is found to go
hand-in-hand with their radiative power. The good radiator is a good
absorber, and the bad radiator is a bad absorber. From all this it is
evident that as regards the radiation and absorption of non-luminous
heat, colour teaches us nothing; and that even as regards the
radiation of the sun, consisting as it does mainly of non-luminous
rays, conclusions as to the influence of colour may be altogether
delusive. This is the strict scientific upshot of our researches. But
it is not the less true that in the case of wearing apparel--and this
for reasons which I have given in analysing the experiments of
Franklin--black dresses are more potent than white ones as absorbers
of solar heat.

Thus, in brief outline, have been brought before you a few of the
results of recent enquiry. If you ask me what is the use of them, I
can hardly answer you, unless you define the term use. If you meant
to ask whether those dark rays which clear away the Alpine snows, will
ever be applied to the roasting of turkeys, or the driving of
steam-engines--while affirming their power to do both, I would frankly
confess that they are not at present capable of competing profitably
with coal in these particulars. Still they may have great uses
unknown to me; and when our coal-fields are exhausted, it is possible
that a more aethereal race than we are may cook their victuals, and
perform their work, in this transcendental way. But is it necessary
that the student of science should have his labours tested by their
possible practical applications? What is the practical value of
Homer's Iliad?