The effect of water vapour on the strength of fused quartz has been investigated by measuring the bending strength S, at 20 and 40 $^\circ$C, of microscope cover slides in varying humidities (p/p$_0$) relative to the bending strength in vacuo S$_0$, and correlating this with the adsorption of water vapour on crushed slides at the same p/p$_0$ and temperatures. S/S$_0$ was found to vary with p/p$_0$ in the anomalous way previously observed for soda-lime glass. While an overall reduction in strength of about one-third occurred, a restoration in strength at intermediate pressures was observed at 20 $^\circ$C. This restoration was not observed at 40 $^\circ$C. These phenomena, combined with the reversible and irreversible nature of the adsorption of water vapour on the surface of the powder at the same p/p$_0$, provided a means of interpreting the surface reactions between water and fused quartz. A model of these reactions is proposed which led to a prediction that a delay in failure with a further reduction in strength should be observed if the time of load application is increased from a few minutes to a number of hours. Subsequent experiments confirmed this prediction.