Another method, very old in principle, has also lately acquired great
importance. For a long time we sought to estimate the temperature of a
body by studying its radiation, but we did not know any positive
relation between this radiation and the temperature, and we had no
good experimental method of estimation, but had recourse to purely
empirical formulas and the use of apparatus of little precision. Now,
however, many physicists, continuing the classic researches of
Kirchhoff, Boltzmann, Professors Wien and Planck, and taking their
starting-point from the laws of thermodynamics, have given formulas
which establish the radiating power of a dark body as a function of
the temperature and the wave-length, or, better still, of the total
power as a function of the temperature and wave-length corresponding
to the maximum value of the power of radiation. We see, therefore, the
possibility of appealing for the measurement of temperature to a
phenomenon which is no longer the variation of the elastic force of a
gas, and yet is also connected with the principles of thermodynamics.
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