A system of physical chemistry - Index of

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A system of physical chemistry - Index of

56 -/ SYSTEM OF PHYSICAL CHEMISTRY

better thermal conduction, and attainment of equilibrium. Nernst also

describes a third form of calorimeter, suitable for liquids, details of

which will be found in the paper cited. The heat capacity of the fittings

of the calorimeter K— other than the actual substance examined—was

determined, and found to agree well with that calculated from the

known masses and specific heats of the substances employed. When

the pear-shaped vessel was well evacuated, the temperature of the sub-

stance, i.e. the calorimeter K (as measured by the resistance of the

spiral) was found to remain remarkably constant for a considerable time,

even when K had been previously cooled down to very low temperatures,

by placing the non-evacuated pear-shaped vessel in contact with liquid

air. Similarly, after cutting off the current the new (final) temperature

was again Xound to remain constant, and be accurately measurable.

Nernst considers that the probable error in the results does not exceed

I per cent., and may be much less.

Some of the experimental data, obtained by the above means, have

already been given in dealing with Nernst's heat theorem, and other

examples will be given when we take up in greater detail the question

of the applicability of Einstein's expression to the atomic heat of solids

in general.

Methods ok Determining the Characteristic Vibration

Frequency of a Solid.

First Method.

This depends on the direct measurement of the " Reststrahlen

of selective

(Residual Rays) ". Many substances possess the property

reflection, that is they powerfully reflect rays of certain wave-length.

The rays which are most strongly reflected in the case of incident light

are those which are most strongly absorbed when the light is transmitted.

The wave-lengths strongly absorbed are identical with the characteristic

of the

wave-lengths corresponding to the natural vibration frequencies

substance, for it is when the light has the same frequency as that of the

vibrating atom that it is most strongly absorbed. If a beam of incident

light of all wave-lengths, that is to say, a beam emitted by a heated

black body, is reflected successively from a number of pieces of the

solid in question, at each reflection the beam will become purer, that is

the reflected light tends more and more to consist of the vibration

frequency characteristic of the solid. The rays which survive after

having suffered a number of such reflections are called the " Residual

Rays ". The measurement of the wave-length and the energy of such

Reststrahlen has been principally carried out by Prof. Rubens of Berlin

and his collaborators, although the idea was that of Beckmann in the

first instance {cf. Rubens and Nichols, Annalen der Physik, 60, 418,

1897 ; Rubens and Aschkinass, ibid., 65, 241, 1898 ; Rubens, ibid., 69,

576, 1899; Rubens and Kurlbaum, ibid., [4], 4, 649, 1901 ; Rubens

and Hollnagel, Phil. Mag., [6], 19, 761, 1910). These rays correspond-

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