1. The conditions of photogel formation and reversion in rubber solutions have been studied. The larger the rubber molecule the more readily is gel formation effected. 2. Photogels exhibit all the properties associated with cross-linked molecules. 3. Photogels made with most synthetic rubbers are not influenced to the same extent by the presence of oxygen, and do not revert on further exposure in the presence of oxygen. 4. The suggestion is made that gel formation is due to oxygen-bridging of already oxygenated centers. This bridging is not possible in the presence of oxygen, since further oxidation results, with cleavage of the molecule. In the case of active solvents, bridging may be due primarily to condensation with solvent. Bridging is also probably accompanied by a limited degree of cyclization.

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