190 FRAGMENTS OF SCIENCE. 



tensity 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 cur 

 rents of air ; through an orifice in the globe the rays could 

 pass from the spiral and fall afterward upon a thermo-elec 

 tric 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 



of spiral. obscure radiation. 



Dark 1 



Dark, but hotter 3 



Dark, but still hotter 5 



Dark, but still hotter 10 



Feeble red 19 



Dull red 25 



Red 37 



Full red 62 



Orange 89 



Bright orange 144 



Yellow 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 four hundred 

 and forty 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 



