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l 90 FRAGMEN TS OF SCIEN CE.

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

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