the greater part of the vapour, this is because the dissocia- tion begins even below the boiling point of the substance. Further, we know more than one substance whose vapour density does not become abnormal until the tem- perature is considerably above its boiling point. This is the case with the compounds of amylene with hydrochloric aeid, hydrobromic acid, and hydriodio acid, and the progress of the dissociation can be followed step by step. The vapour density is at first normal, then becomes inter- mediate, and finally assumes only one half the normal value, corresponding to an almost complete dissociation of OsE^ X B, into C 5 H l0 and HR. Similarly in the case of iodine, the molecule I 2 remains undecomposed up to a temperature of 500°. The dissociation then commences, and at about 1500° the vapour consists for the greater part of isolated iodine atoms.
��Specific Heat of the Elements in the
Solid State
Dulong and PetiVs Law
The application of Avogadro's hypothesis has enabled us to ascertain the atomic weights of a certain number of elements, in particular of the comparatively light elements and those of a non-metallic character. But it leaves us in ignorance of the atomic weights of the majority of the heavier metallic elements.
Dulong and Petit observed that the specific heat of an element is inversely proportional to its atomic weight, or, in other words, the atomic heat — which is the product of specific heat and atomic weight — has a constant value for all elements.
The application of this law is, however, limited to the solid elements at temperatures sufficiently below their melting points. Then, taking as unit of specific heat the specific heat of liquid water, and as unit of atomic weight .
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