A system of physical chemistry - Index of

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A system of physical chemistry - Index of

APPENDIX II.

Foundations of the Quantum Theory.

By J. Rice, M.A.

(From the Transactions of the Faraday Society^ xi., p. i, 1915.)

During the nineteenth century certain facts had been well established

concerning the radiation contained in an enclosure maintained at constant

temperature. Balfour Stewart and Kirchhoff had shown that if

the material of the walls was not perfectly reflecting for any quality of

radiation, any constituent of the radiation having a definite frequency

was present in an amount depending on that frequency, the temperature

and size of the enclosure, but independent of the nature of the wall-

material. Stefan and Boltzmann had proved that the energy of the

total radiation in one c.c. was proportional to the fourth power of the

absolute temperature. Wien had reached the conclusion that the

energy-density of those constituents of the radiation, whose wave-lengths

lay between narrow limits X and X -f 8A, was ^ 8A., where the

function /in the numerator, although undetermined in form, was dependent

on the single variable A.T, the product of the wave-length and

the absolute temperature. These statements had been arrived at by

reasoning of a purely thermodynamic nature, based on the existence

of a radiation pressure. It was only natural that the statistical methods,

already employed as a successful weapon of attack on problems arising

out of the kinetic theory of gases, should be presently pressed into service

in this new line of research. There is some resemblance between

a vessel containing molecules, individually uncontrollable but maintaining

a certain average condition of energy by encounters, and an enclosure

maintaining a statistically permanent condition of radiation by

the emission and absorption of its walls, while the history of any particular

wave-train of definite frequency cannot be followed in detail.

There is one marked difference between the two cases : molecules can,

by direct encounter with each other, as well as by collision with the

walls, exchange energy ; but in a temperature enclosure passage of

energy from one wave-train to another must take place through the

agency of the walls alone or of matter contained within them, since two

such trains can pass through the same element of volume {i.e. cross one

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