RADIATION. 39 
every other fay 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 cm-rents 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 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: 
