have acquired as high a temperature as they can
attain in this position. Which of the cards is then most highly
heated? It requires no thermometer to answer this question. Simply
pressing the back of the card, on which the white powder is strewn,
against the cheek or forehead, it is found intolerably hot. Placing
the dark card in the same position, it is found cool. The white
powder has absorbed far more heat than the dark one. This simple
result abolishes a hundred conclusions which have been hastily drawn
from the experiments of Franklin. Again, here are suspended two
delicate mercurial thermometers at the same distance from a gas-flame.
The bulb of one of them is covered by a dark substance, the bulb of
the other by a white one. Both bulbs have received the radiation from
the flame, but the white bulb has absorbed most, and its mercury
stands much higher than that of the other thermometer. This
experiment might be varied in a hundred ways: it proves that from the
darkness of a body you can draw no certain conclusion regarding its
power of absorption.

The reason of this simply is, that colour gives us intelligence of
only one portion, and that the smallest one, of the rays impinging on
the coloured body. Were the rays all luminous, we might with
certainty infer from the colour of a body its power of absorption; but
the great mass of the radiation from our fire, our gas-flame, and even
from the sun itself, consists of invisible calorific rays, regarding
which colour teaches us nothing. A body may be highly transparent to
the one class of rays, and highly opaque to the other. Thus the white
powder, which has shown itself so powerful an absorber, has been
specially selected on account of its extreme perviousness to the
visible rays, and its extreme imperviousness to the invisible ones;
while the dark powder was chosen on account of its extreme
transparency to the invisible, and its extreme opacity to the visible,
rays. In the case of the radiation from our fire, about 98 per cent
of the whole emission consists of invisible rays; the body, therefore,
which was most opaque to these triumphed as an absorber, though that
body was a white one.

And here it is worth while to consider the manner in which we obtain
from natural facts what may be called their intellectual value.
Throughout the processes of Nature we have interdependence and
harmony; and the main value of physics, considered as a mental
discipline,