A system of physical chemistry - Index of

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A system of physical chemistry - Index of

92

A SYSTEM OF PHYSICAL CHEMISTRY

temperature and converges to zero at absolute zero of temperature. In

the special case of a monatomic molecule in which the precessional

vibrations are exceedingly rapid the corresponding quantum must be

exceedingly great, and consequently the likelihood of any molecule

possessing even one such quantum is negligibly small. In other words,

the observed energy content can only be due to the free translational

motion which is always directly proportional to the absolute temperature,

i.e. the molecular heat in such a case is 3/2R, in complete agreement

with experiment. In the case of diatomic gases Kriiger has shown

that the vibration frequencies of precession correspond to the farther

infra-red region, about 30/A in the case of hydrogen, and that the so-

-called rotation is spectrum really due to precessional vibrations. It is

only at high temperatures that the precessional vibration becomes so

violent as to merge into a rotation {cf. Chap. It must be I.).

clearly

understood that the conflicting views of Bjerrum and Kriiger have

nothing to do with the ordinary atomic to-and-fro vibration inside the

molecule, which Bjerrum has shown gives rise to bands in the short

infra-red i.e. region, at wave-lengths shorter than lo/x.

On the whole, Kriiger's view serves to bring the behaviour of monatomic

and diatomic gases into much closer accord than had hitherto

been the case. Considerable experimental work is still necessary, how-

ever, before we can decide between the theory of molecular rotations

and the theory of precessional vibrations.

A translation of the major part of Kriiger's papers will be found in

Appendix III. for those desirous of greater

following Kriiger's treatment in

detail.

The Molecular Heat of Hydrogen Gas at Low Temperatures.

{Cf. A. Eucken, Sitzungsber. kon. preuss. Akad. Wissenschaft ,

p. 141, 1912.)

The molecular heat of hydrogen is of particular interest because

hydrogen is the simplest type of diatomic molecule. The most striking

result obtained by Eucken is that at low temperatures hydrogen behaves

as a monatomic gas, i.e. it exhibits no energy of a rotational or of a

gyroscopic kind. The experimental details will be found in the paper

referred to. It is sufficient in the present instance to quote certain of

the results. Eucken employed different quantities of hydrogen in the

calorimeter (internal volume, 39 c.cs.), and his experiments are sufficiently

exact to show that the molecular heat varied with the concentration

of the gas. The following table gives the value of the molecular

heat observed corresponding to a particular temperature,^ and the mass

of hydrogen employed :—

^ The molecular heat values are " instantaneous," or true molecular heats at a

given temperature; not mean values over a temperature range.

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