Thus is
explained the phenomenon of the emission of radiations. In the same
way, the movement of electrons may be excited or modified by the
electrical forces which exist in any pencil of light they receive, and
this pencil may yield up to them a part of the energy it is carrying.
This is the phenomenon of absorption.
Professor Lorentz has not contented himself with thus explaining all
the mechanism of the phenomena of emission and absorption. He has
endeavoured to rediscover, by starting with the fundamental
hypothesis, the quantitative laws discovered by thermodynamics. He
succeeds in showing that, agreeably to the law of Kirchhoff, the
relation between the emitting and the absorbing power must be
independent of the special properties of the body under observation,
and he thus again meets with the laws of Planck and of Wien:
unfortunately the calculation can only be made in the case of great
wave-lengths, and grave difficulties exist. Thus it cannot be very
clearly explained why, by heating a body, the radiation is displaced
towards the side of the short wave-lengths, or, if you will, why a
body becomes luminous from the moment its temperature has reached a
sufficiently high degree.
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