The strength of glass was measured in various surrounding media. It was found that glass is 20 percent stronger when dry than when wet, and two to two and a half times as strong when baked in vacuum than when tested wet (on tests of ten seconds duration). Apparently, the chief cause of the loss of strength, as compared with that in vacuum, is moisture, but gases, especially CO2, seem to have some effect. It was found that fatigue of glass disappears when the glass is tested in a vacuum.Autoclaving tests showed that glass is rapidly etched by water at higher temperatures, and that even silica glass is considerably attacked. It appears that the attack of water on glassy surfaces produces gels, and this attack goes on inside the flaws, thus considerably complicating the problem of the strength of glass.