## Abstract

The measurements were made with two adiabatic calorimeter assemblies, one for the temperature range 2$\cdot $8 to 20 degrees K and the other for the range 12 to 270 degrees K. The working thermometer for the former assembly was a carbon resistor and for the latter a strain-free capsule-type platinum resistance thermometer. The crystals (pieces approximately 0$\cdot $5 cm in dimension) were sealed in thin copper calorimeter vessels with a small amount of helium gas to promote heat exchange. The accuracy of the measurements is estimated to be $\pm $2% at the lowest temperatures, rising rapidly to $\pm $ 0$\cdot $2% for T $\geq $ 20 degrees K. The precision was generally better than this by about a factor of two at most temperatures. Apparent Debye characteristic temperatures ($\Theta _{D}$) have been calculated for all of the experimental results. The variation of $\Theta _{D}$ with temperature is in general accord with the predictions of theory. Starting from the lowest temperatures, $\Theta _{D}$ for the three potassium salts first decreases to a minimum in the region of T = $\Theta _{D}$/12, then increases to a maximum in the region of T = $\Theta _{D}$/2 and subsequently decreases smoothly. $\Theta _{D}$ for sodium iodide shows the same behaviour at low temperatures but is still increasing at 270 degrees K.