Page:Outlines of Physical Chemistry - 1899.djvu/39

 melting points. The case of carbon seems to show that there is also a lower limit of temperature. Indeed, the specific heat of carbon, as well as that of boron and silicon, increases comparatively rapidly as the temperature rises. If the rate of increase, which has been observed between 0° and 200°, were maintained above this temperature, the atomic heat of carbon would become normal at about 500°. In practice, Dulong and Petit 1 s law, which, as we have seen, is applicable to the heavy elements and in general to all metals, is made the basis of determination of atomic weights for those elements where, by reason of the absence of volatile compounds, Avogadro's hypothesis is unable to render assistance. For the determination of the atomic weights of the non-metals or of elements of low atomic weight a study is always made of the volatile derivatives.

Molecular Heat (Neumann)

The atomic heat of the elements is maintained without modification in their compounds. The molecular heat of a substance is equal to the sum of the atomic heats of all the atoms contained in the molecule. We thus meet here what is termed an additive property of matter. For example :

The molecular heat of lead bromide, PbBr 2, found by experiment

is 19-9.

That found by calculation is 6*5 + 2 x 67 = 19*9.

From the experimentally found molecular heat of lead chloride, PbCl 2, the atomic heat of chlorine can be deduced. It is 5*95.

The investigation of molecular heats of compounds has rendered it possible to derive the following atomic heats :

��H.

�C.

�O.

�Si.

�F.

�P.

�N.

�S. -.

�B.

�� �