A system of physical chemistry - Index of

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A system of physical chemistry - Index of

1 82 A SYSTEM OF PHYSICAL CHEMISTRY

should have to postulate the existence of a very great number of oscil-

lators of different frequencies. Now, all the evidence which we possess

on the nature of atomic structure negatives such a view, nor is there

any real evidence that any vibrating electron in an atom emits simple

harmonic waves at all. Indeed, one of the outstanding difficulties of

the Planck theory is just this difficulty of formulating a plausible

physical basis for it.

There is a further difficulty awaiting us when we seek to apply the

notion of discontinuous absorption to the phenomena of ejection of

electrons from the surfaces of bodies on which is incident a beam of

ultra-violet light (the photo-electric effect), or a pencil of X-rays. All

the evidence favours the view that a definite amount of energy has to be

at least resident in the electron before it can be ejected. In the normal

photo-electric effect the energy of the electron after is escape found to

have a maximum value h'v - zv, where vis the frequency of the incident

light, h' a constant whose value agrees fairly closely with Planck's h,

and ze; is a constant characteristic of the metal from which the electrons

are escaping. A quite plausible hypothesis is that h'v represents the

amount of energy in the electron before its escape; of this it loses an

amount w in leaving the atom against restraining forces which depend

on the particular structure of the atom ; and so, if it is on the surface

and has not to part with still more energy in escaping from the influence

of neighbouring atoms, it would retain an amount h'v — w after emer-

gence. Now this appears to fit very well indeed with Planck's original

assumption of the discontinuous and sudden absorption of a quantum

hv, which,' if v is large enough, would supply the necessary energy for

the escape of the electron from those atoms which the laws of chance

brought at any definite instant into the suitable condition for absorption.

But there seems to be this fatal objection to such a simple hypothesis,

that the photo-electric effect has been observed zvithout any delay in

cases where the intensity of the light, as calculated by the inverse

square law and on the assumption of continuous propagation through

the ether, could not have possibly supplied the necessary quantum to an

electron in a molecule or atom in less than lo or 15' minutes. Similar

difficulties arise in connection with the secondary /?-rays caused by

X-rays, if one considers the energy of the X-ray pulse as uniformly

distributed throughout the pulse. In fact, if one adheres to the

hypothesis of discontinuous absorption, one must admit that the energy

of any beam of radiation is not uniformly diffused throughout the space

occupied by the beam. This is indeed roughly the manner in which

Einstein escapes from the difficulties. He has advanced the view that

the energy of light is itself atomic in structure, and absorption and

emission are of necessity discontinuous, not because of the special

structure of the oscillators, etc., but because of the nature of light itself.

His view is, in reality, a quantum of energy view; it has already been

observed that Planck's is not necessarily so ; indeed Planck dissociates

himself at present from so revolutionary a hypothesis. It is noteworthy

that J. J. Thomson had already been led to speculations somewhat

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