RADIATION. 185 



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 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 Hays : 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, " Philosophical Maga- 

 zine," vol. xii., p. 521. 



