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The effect of water diffusion on the strength of silica glass is discussed. When silica glass is immersed in hot water and held there for an extended period, its strength increases over that of freshly damaged glass. We propose that this increase in strength is due to water diffusion into exposed surfaces of the test specimen, resulting in swelling of the glass and a fracture mechanics shielding of cracks present in the glass surface. In the present paper, we show that the shielding may be broken down into two parts, that associated with the crack itself and that associated with the free surface of the specimen. The explanation proposed here has been neglected in earlier studies. The approach allows us to obtain a reasonable quantitative explanation for the strengthening of silica glass by soaking in hot water.