A system of physical chemistry - Index of

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A system of physical chemistry - Index of

CHAPTER IL

Introductory — Planck's concept of quanta — Planck's radiation formula.

The fundamental modification introduced by Planck consists in dis-

carding the principle of ^^?«partition of kinetic energy. The equipartition

principle assumes interchange of energy in a continuous

.manner, i.e. without any lower limit to the amount of energy transferable.

Planck assumes, on the other hand, that the exchanges of

energy between matter and ether, instead of taking place in any proportion

whatsoever, can only take place by steps, that is, in multiples

of some small energy unit, the energy unit itself being a function of

the vibration frequency concerned. We might regard radiant energy

itself as possessing a structure. Planck himself regards the matter from

a different standpoint. It would be more correct to say that the de-

finite concept of radiant energy itself being discrete in nature is that

upheld by Einstein, a view which has been given a clearer physical

basis by J. J. Thomson, who considers a radiant-energy unit or

" quantum " as a region of periodic disturbance travelling along a

Faraday tube. In place of a continuous ether, we have therefore to

substitute a number of stretched strings of ether, each string being a

Faraday tube differentiated in some (practically unknown) way from

the

" "

it. space surrounding This statement, is, however, to be

taken rather as a rough material analogy than as an exact description

or formulation, for any exact description is at the present time impossible.

Planck, on the other hand, lays stress not on the question of

the ultimate structure of radiant energy itself so much as on the mode

of its absorption and emission by matter. The energy radiated by one

element of a black body is partially absorbed by other elements.

Each one of the vibrators, or " resonators," as Planck calls them, which

constitute the material element in question, can only emit (and absorb)

energy in certain fractions. (This at any rate is Planck's first position ;

we shall see later that he has modified the above view.) A word here

about the Planck resonators. It was pointed out in the chapter on

photochemistry in Vol. II. that the vibrating particles which produced

visible and ultra-violet light were much smaller than the atoms of the

substance and were therefore in all probability the electrons. The

amount of energy contributed to the "total" spectrum by the visible

or ultra-violet region has been shown by experiment {cf. Lummer and

35 3*

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