RADIATION. 35 
not already Black, ought to be blackened. When metals 
are to be burned, it is necessary to blacken or otherwise 
tarnish them, O as to diminish their reflective power. 
Blackened zinc foil, when brought into the focus of invisi- 
ble rays, is instantly caused to blaze, and burns with its 
peculiar purple light. Magnesium wire flattened, or tar- 
nished magnesium ribbon, also bursts into flame. Pieces 
of charcoal suspended in a receiver full of oxygen are also 
set on fire when the invisible focus falls upon them; the 
dark rays after having passed through the receiver, still 
possessing sufficient 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: * 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 solution, 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 pecul- 
iar 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 paper, 
two holes are pierced in it, corresponding to the images of 
the two coke points: but falling on a thin plate of carbon 
in vacuo, or upoii a thin sheet of platinized platinum, 
either in vacuo or in air, radiant heat is converted into 
light, and the image stamps itself in vivid incandescence 
upon both the carbon and the metal. Results similar to 
those obtained with the electric light have also been ob- 
tained with the invisible rays of the lime-light and of the 
sun. 
Before a Cambridge audience it is hardly necessary to 
*I borrow this terra from Professor Chain's, "Philosophical Maga- 
zine," vol. xii., p. 521. 
