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Would show no preference for black over white; but they
do show a preference, and, to obtain rapid combustion, the
body, if not already black, ought to be blackened. When
metals are to be burned, it is necessary to blacken or
otherwise tarnish them, so as to diminish their reflective
power. Blackened zinc-foil, when brought into the focus
of invisible rays, is instantly caused to blaze, and burns
with its peculiar purple flame. Magnesium wire flattened,
or tarnished magnesium ribbon, also bursts into splendid
combustion. Pieces of charcoal suspended in a receiver
full of oxygen are also set on fire: the dark rays after hav-
ing passed through the receiver still possessing suflicient
power to ignite the charcoal, and thus initiate the attack
of the oxygen. If, instead of being plunged in oxygen,
the charcoal be suspended in vacuo, it immediately glows
at the place where the focus falls.

8. Transmutation of Rays .' 1 Calorescence.

Eminent experimenters were long occupied in demon-
strating the substantial identity of light and radiant heat,
and we have now the means of offering a new and striking
proof of this identity. A concave mirror produces beyond
the object which it reflects an inverted and magnified image
of the object; withdrawing, for example, our iodine solu-
tion, an intensely luminous inverted image of the carbon
points of the electric light is formed at the focus of the
mirror employed in the foregoing experiments. When the
solution is interposed, and the light is cut away, what
becomes of this image? It disappears from sight, but an
invisible thermograph remains, and it is only the peculiar
constitution of our eyes that disqualifies us from seeing
the picture formed by the calorific rays. Falling on
white paper, the image chars itself out: falling on black

1 I borrow this term from Professor Challis, “ Philos0phica1 Maga-
zine,” vol. xii., p. 521.

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