The sorption of ammonia by glass wool at 0°C. and at different pressures was investigated by Bangham and Burt. The glass wool, after being cleaned in situ with chromic acid and washed with water, was out-gassed at 200° C. After each experiment the ammonia was expelled from the sorbent by heating to this temperature while part of the system was cooled with liquid air. Under these conditions reproducible results were obtained, and it was found possible to express the sorption at any time during an experiment as a function of the pressure and the time from the start. A small correction was needed to take account of the diminution of pressure as sorption progressed. An outstanding feature of the phenomenon investigated was the extraordinary slowness of the later stages of the process, as a result of which the experimental attainment of equilibrium proved quite impracticable. When the logarithms of the quantities of gas taken up were plotted against the logarithms of the time from the start, the resulting graphs were nearly linear over their whole course, and of very small gradient (about 1/12), a large fraction of the sorption being completed before any observations could be taken. It therefore proved impossible to assess limiting values, and the present work was undertaken with the hope of throwing more light on this question. Regarded as a sorbent, glass is a highly unsatisfactory material, mainly because of the impossibility of freeing it from water. Since, however, the primary object of these investigations has been to find the order of correction that should be applied for sorption effects in glass apparatus used for accurate gas measurement, we have submitted our sorbent to such heat treatment as would be feasible, say, in the case of a calibrated density bulb, instead of aiming at the highest possible degree of desiccation. The tenacity with which water and traces of gas are held, even at far higher temperatures, is illustrated in a recent paper by Durau. That the water content of glass can be kept very nearly constant by careful regulation of the temperature of out-gassing has been established both by the work of Sherwood and by the earlier experiments carried out in this laboratory, and finds further confirmation in the present research.