68 A SYSTEM OF PHYSICAL CHEMISTRY The agreement between theoretical and observed values is good. The important point about the diamond, as already remarked, is that it ceases to have any measurable heat capacity at all for about 40° above absolute zero. This is in agreement both with the thermo- dynamic theorem of Nernst and the Einstein theory of specific heat. We now pass to the consideration of the salts KCl, NaCl, and KBr. Each of these possesses a single well-defined band in the far infra red measured experimentally by Rubens. This value has been employed in the equation. The values of C„ and C/. are mean atomic heats, i.e. one half the molecular heats. T Absolute. KCl. /?oi' =218.
ATOMIC HEATS OF SOLIDS 69 KBr. /Sqv =177 (C/- was calculated from C„ by the usual formula, the value of A being taken as identical with that for NaCl). T Absolute.