A system of physical chemistry - Index of

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A system of physical chemistry - Index of

54

A SYSTEM OF PHYSICAL CHEMISTRY

as far as the diamond is concerned, Einstein's theory reproduces experimental

values with very considerable fidelity. While giving full weight

to such general agreement, it is necessary to point out that this agreement

is far from being complete in many other cases. We shall return

to this after having described in outline the experimental methods of

determining C^, and the characteristic vibration frequency v.

Experimental Measurements of the Specific Heats of Solids, especially

at Low Temperature.

{Cf Nernst, Journ. de Physique, [4], 9, 19 10; Nernst, Koref, and

Lindemann, Sitzungsher. Berl. Akad., 1910, vol. i, p. 247 ; ibid., Nernst,

p. 262 ; Nernst, Annalen der Physik, [4], 36. 395) 1911-)

One form of calorimeter consisted of a heavy vessel of copper (about

400 grams in weight), the good thermal conductivity of such a mass

of copper doing away with the necessity of stirring the substance, which

is, of course, impossible in the case of solids. This was enclosed in a

Dewar vacuum vessel, placed in a bath, the temperature of the calorimeter

being measured by means of thermo-couples. The substance to

be investigated was heated or cooled to a known temperature, and introduced

into the calorimeter, the change in temperature of which was

observed. Knowing the heat capacity of the calorimeter, the specific

heat of the substance could be obtained. This method worked admirably,

but of course it is limited to the determination of mean values of

Ci,, hoMing over a considerable temperature range. For the purposes

in view, it was necessary, however, to be able to determine C„ for small

temperature ranges, that is for a consecutive series of "points " on the

temperature scale. To accomplish this, a different procedure had to

be adopted.

The principle of this second method consists briefly in making the

investigated substance itself act as its own calorimeter. The substance

could be heated electrically by means of a platinum spiral, through

which a known quantity of electrical energy was passed, as measured

by an ammeter and voltmeter in the circuit. The rise in temperature

of the substance was obtained by using the heating spiral

itself as a

resistance thermometer, i.e. its resistance was observed by means of a

bridge, before and after the heating of the substance, the alteration in

resistance giving the temperature change. Knowing the mass of substance

employed, the electrical energy supplied, and the rise in temperature,

it is easy to calculate the specific heat of the substance. This

extremely ingenious method was worked out by Eucken, in Nernst's

laboratory {Physik. Zeitschr., 10, 586, 1909), and has been employed

by Nernst and his collaborators for the accurate determination of specific

heats of substances at various temperatures, extending over a wide range,

even down to the temperature of liquid hydrogen. A few details may

be given here. A pear-shaped vessel of glass (shown in the accompanying

figure, Fig. 6) contains the calorimeter K suspended by the

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