Thermal conductivities of from 2 to 500×10−4c.g.s. units are measured by comparing the flow of heat through a given sample with that through a standard material. On the basis of repeat runs and checks with literature values, an accuracy of about five percent is estimated for this method. The dimensions of the apparatus limit the face size of both the unknown and standard samples to a maximum of 10 millimeters square; thicknesses of 2 to 10 millimeters are suitable. Values at average temperatures between 30°C and 75°C are reported for nine materials:—sodium chloride, potassium chloride, potassium bromide, calcium fluoride, lithium fluoride (both air‐grown and vacuum‐grown), silver chloride, thallium bromide‐iodide, thallium bromide‐chloride, and fused silica.