48 FRAGMENTS OF SCIENCE. 



8. Transmutation of Rays : l Caloreseence. 



Eminent experimenters were long occupied in de- 

 monstrating 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 in- 

 tensely 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 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 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. Results 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 



1 I borrow this term from Professor Challis, 'Philosophical 

 Magazine,' vol. xii. p. 621. 



