18 OUTLINES OF PHYSICAL CHEMISTEY
that of hydrogen, the atomic heat of the elements generally
lies between the values 5 and 7, and is nearly 6*4.
For example : and the atomic weight is known to be 79*75.
Therefore the atomic heat is 0-0843 x 79*75 = 67. In more concrete terms it may be stated that to raise the temperature of one gram-atom (79*75 grms.) of solid bromine through one degree of temperature, 6*7 small calories (gram-units of heat) are required.
A large number of other elements, whose atomic weights we already know (Al, P, Si, Ti, Cr, Zn, Ga, Ge, As, Se, Br, Zr, Mo, Cd, In, Sb, Sn, Te, I, W, Os, Hg, Tl, Pb, Bi), have an atomic heat which falls within the above-mentioned limits. The atom of these elements is therefore that quantity of matter to which a constant quantity of heat must be imparted in order to produce the same rise of temperature.
For the numerous elements for which Avogadro's hypothesis is not available, we take as atomic weight that multiple of the chemical equivalent which best satisfies the condition :
Atomic weight x specific heat = 6*4.
By this method of investigation, the atomic weights of the following elements have been determined : Li, Na, Mg, K, Ca, Mn, Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, La, Ce, Ir, Pt, Au, Th, and U.
Certain elements of low atomic weight have atomic heats which are much too small and do not at all agree with JDulong and PetiVs law. To this class boron and carbon belong. Silicon agrees better, sulphur and phos- phorus have almost normal atomic heats. The values for the light metals (Li, Na, K) are absolutely normal.
Dulong and PetiVs law, like every other physical law, is only applicable within certain limits. It is only true for solids at temperatures sufficiently distant from their
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