The thermal conductivity of air is determined in absolute measure between -183 and 218 °C. by the " thick-wire " variant of the " hot-wire " method. The apparatus consists of a platinum wire 11 6 cm. long and 1.5 mm. in diameter mounted in a 90 per cent. Pt 10 per cent. Ir tube of 7 mm. internal diameter. The temperature variation of the thermal conductivity of air can be represented with considerable accuracy by the quadratic formula               kt=5. 75 x 10-5(l +0.00317t-0.0000021t2) The present paper suggests a value for the thermal conductivity of air at 0 °C. from a consideration of the data which have been obtained by various hot-wire methods.