RADIATION. 39 



every other ray of the spectrum, visible and invisible. We 

 cannot indeed see the augmentation of intensity in the 

 region beyond the red, but we can measure it and express 

 it numerically. With this view the following experiment 

 was performed: A spiral of platinum wire was surrounded 

 by a small glass globe to protect it from currents of air; 

 through an orifice in the globe the rays could pass from 

 the spiral and fall afterward upon a thermo-electric pile. 

 Placing in front of the orifice an opaque solution of iodine, 

 the platinum was gradually raised from a low dark heat to 

 the fullest incandescence, with the following results: 



Appearance Energy of obscure 



of spiral. radiation. 



Dark 1 



Dark, but hotter 3 



Dark, but still hotter . 5 



Dark, but still hotter 10 



Feeble red 19 



Dullred 25 



Red 37 



Full red '. 62 



Orange , 89 



Bright orange 144 



Yellow s 202 



White 276 



Intense white , 440 



Thus the augmentation of the electric current, which 

 raises the wire from its primitive dark condition to an in- 

 tense white heat, exalts at the same time the energy of the 

 obscure radiation, until at the end it is fully 440 times 

 what it was at the beginning. 



What has been here proved true of the totality of the 

 ultra-red rays is true for each of them singly. Placing our 

 linear thermo-electric pile in any part of the ultra-red spec- 

 trum, it may be proved that a ray once emitted continues 

 to be emitted with increased energy as the temperature is 

 augmented. The platinum spiral, so often referred to, 

 being raised to whiteness by an electric current, a brilliant 

 spectrum was formed from its light. A linear thermo- 

 electric pile was placed in the region of obscure rays be- 

 yond the red, and by diminishing the current the spiral 

 was reduced to a low temperature. It was then caused to 

 pass through various degrees of darkness and incandes- 

 cence, with the following results: 



