The thermal conductivities of a selection of non-metals at 20°C. and 100°C. have been determined by precisely the same method, and in the main the same apparatus as was recently employed by the author in the determination of the "Thermal conductivity of some of the rarer metals and alloys." The substances treated were in the form of accurately turned cylinders, placed in an enclosure at constant temperature. Measurements were made of the heat given (electrically) to one end of the cylinder, and of the temperatures of this end and of the enclosure, and the thermal conductivity deduced from a simple formula. A table of results is given, which agree well with those obtained by Lees and other observers in cases where a direct comparison is available.