RADIATION. 35 



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 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 upon 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 vvith the invisible rays of the lime-light and of the 

 sun. 



Before a Cambridge audience it is hardly necessary to 



*I borrow this term from Professor Challis, "Philosophical Maga- 

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



