Page:EB1911 - Volume 18.djvu/219



Thermal Properties.—The specific heats of most metals have been determined. The general result is that, conformably with Dulong and Petit’s law, the “atomic heats” all come to very nearly the same value (of about 6·4); i.e. atomic weight by specific heat＝6·4. Thus we have for silver by theory 6·4/108=·0593, and by experiment ·0570 for 10° to 100° C.

The expansion by heat varies greatly. The following table gives the linear expansions from 0° to 100° C. according to Fizeau (Comptes rendus, lxviii. 1125), the length at 0° being taken as unity.

The coefficient of expansion is constant for such metals only as crystallize in the regular system; the others expand differently in the directions of, the different axes. To eliminate this source of

uncertainty these metals were employed as compressed powders. The cubical expansion of mercury from 0° to 100° C. is ·018153 = (Regnault) (See .)

Fusibility and Volatility.—The fusibility in different metals is very different, as shown by the following table, which, besides including all the fusing points (in degrees C.) of metals which have been determined numerically, indicates those of a selection of other metals by the positions assigned to them in the table. For practical purposes the volatility of metals may be stated as follows:—

1. Distillable below redness: mercury.

2. Distillable at red heats: cadmium, alkali metals, zinc, magnesium.

3, Volatilized more or less readily when heated beyond their fusing points in open crucibles: antimony (very readily), lead, bismuth, tin, silver.

4. Barely so: gold, (copper).

5. Practically non-volatile: (copper), iron, nickel, cobalt, aluminium; also lithium, barium, strontium and calcium.

In the oxyhydrogen flame silver boils, forming a blue vapour, while platinum volatilizes slowly, and osmium, though infusible, very readily.

Latent Heats of Liquefaction.—Of these we know little. The following numbers are due to Person—ice, it may be stated, being 80. The latent heat of vaporization of mercury was found by Marignac to be 103 to 106.

Conductivity.—Conductivity, whether thermic or electric, is very differently developed in different metals; and, as an exact knowledge of these' conductivities is of great importance, much attention has been given to their numerical determination (see ; and ).

The following table gives the electric conductivities of a number of metals as determined by Matthiesen, and the relative internal thermal conductivities of (nominally) the same metals as determined by Wiedemann and Franz, with rods about 5 mm. thick, of which one end was kept at 100° C., the rest of the rod in a “vacuum” (of 5 mm. tension) at 12° C. Matthiesen’s results, except in the two cases noted, are from his memoir in Pogg Ann., 1858, ciii., 428.