RADIATION 53 



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 : l 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 im- 

 age of the object. Withdrawing, for example, our iodine 

 solution, an intensely luminous inverted image of the car- 

 bon 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 paper, 

 two holes are pierced in it, corresponding to the images 

 of the two coke points: but falling on a thin plate of car- 

 bon 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 



1 I borrow this term from Professor Challis, * 'Philosophical Magazine/' 

 TO!, xii. p. 621. 



