The sound velocities and elastic constants have been measured for pure single crystals of KI and KCl by the ultrasonic pulse technique over the temperature range from 4°K to 300°K. In KI, $c_{11}=3.38\mathrm{}$, $c_{12}=0.22\mathrm{}$, and $c_{44}=0.368\mathrm{}$ in units of ${10}^{11}$ dynes/${\mathrm{cm}}^2$ at 0°K according to our extrapolations from 80°K. In KCl, measurements of all three constants were made over the entire temperature range down to 4°K and their values at 0°K are $c_{11}=4.83\mathrm{}$, $c_{12}=0.54\mathrm{}$, and $c_{44}=0.663\mathrm{}$ in units of ${10}^{11}$ dynes/${\mathrm{cm}}^2$. The Debye characteristic temperature at 0°K, ${\theta }_0$, has been calculated from these very low-temperature values of the elastic constants to give 129°±2°K for KI and 234°±1°K for KCl. The specific heats of KI, KCl, and KBr have been calculated in this paper by using a combination of a Debye and an Einstein term in the expression for the specific heat. These calculations indicate that the agreement between such a simple theory and experiment becomes better as the ratio of ionic radii and masses increases in these three salts. We point out that in the four alkali halides thus far studied in this laboratory, there is evidence that the Cauchy relation, $c_{12}=c_{44}$, is satisfied at about $T={\theta }_0$. No reason is offered for this coincidence.

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