The form of deterioration which we have designated light stiffening can be detected only when the rubber is in the form of a thin film, as it is, for example, in light-weight single-texture proofings. The implications of this work, however, extend far wider than its immediate application to thin proofings, important though this is, and we have no reason to suppose that the surface crazing seen on articles of rubber exposed to sunlight is not essentially the same thing. It is convenient, for reasons which appear later, to distinguish these two forms of deterioration of rubber, even though chemically and energetically they may have the same fundamental basis. Light stiffening is the noticeable increase in the bending modulus of thin sheets of rubber which occurs with exposure to light and air. This increase is easily observed by handling proofed fabrics so exposed, and in normal speech we refer to a “stiffening” of the article. It is an effect greater at the surface and decreasing with penetration into the bulk rubber, and for this reason is subjectively evident only where the “surface layers” form an appreciable proportion of the total thickness of the rubber. This deterioration becomes essentially a defect confined to thin proofings and other articles, the rubber parts of which form a sheet or layer a few thousandths of an inch in thickness.