We have measured the thermal conductivity of polymethylmethacrylate (PMMA), amorphous ${\mathrm{As}}_{2}$${\mathrm{S}}_{3}$, Ca-K nitrate glass, three ${\mathrm{SiO}}_{2}$-based glasses, and glycerol in the temperature range 30--300 K, using an ac technique that eliminates errors from blackbody radiation. This technique and the problems of blackbody radiation in thermal measurements are discussed in detail. Our data do not support a recent prediction of heat transport by fractons. Instead, we find that the thermal conductivity of these glasses above \ensuremath{\sim}50 K is well described by the minimum thermal conductivity suggested by Slack.