Experimental results are presented giving the Rayleigh number at which convection initiates in a closed cell and the Nusselt number versus Rayleigh number relationship which prevails after convection is initiated. Over 700 data points were obtained for two types of cells heated from below, one a phenolic-fiberglass hexcell of low thermal conductivity and the other aluminum hexcell of high thermal conductivity. Relations based upon the Malkus-Veronis power integral technique and a concept of an equivalent wave number are shown to give good correlation of the experimental data for both the low and high conductivity side walls.