48 FRAGMENTS OF SCIENCE. 



8. Transmutation of Rays:* Calorescence. 



Eminent experimenters were long occupied in de- 

 monstrating the substantial identity of light and radi- 

 ant 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 in- 

 verted and magnified image of the object. Withdraw- 

 ing, 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 em- 

 ployed in the foregoing experiments. When the solu- 

 tion is interposed, and the light is cut away, what be- 

 comes 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: fall- 

 ing on black paper, two holes are pierced in it, corre- 

 sponding to the images of the two coke points: but 

 falling on a thin plate of carbon in vacuo, or upon a 

 thin sheet of platinised platinum, either in vacuo or in 

 air, radiant heat is converted into light, and the image 

 stamps itself in vivid incandescence upon both the car- 

 bon and the metal. Eesults similar to those obtained 

 with the electric light have also been obtained with 

 the invisible rays of the lime-light and of the sun. 



Before a Cambridge audience it is hardly necessary 

 to refer to the excellent researches of Professor Stokes 

 at the opposite end of the spectrum. The above re- 

 sults constitute a kind of complement to his discoveries. 

 Professor Stokes named the phenomena which he has 



* I borrow this term from Professor Challis, ' Philosophical 

 Magazine,' vol. xii. p. 521. 



