Fractures in plate glass, originating at a cutter mark and progressing slowly parallel to the surface, were observed under controlled conditions. The rates of spread of such fractures, immediately after the glass is cut and for several hours afterward, were determined by viewing microscopically the movement of interference fringes produced in monochromatic light. Effects of external atmosphere, and of various included gases introduced during melting, are described. A theory of the kinetics of slow fracture rates is offered, which relates the fracture spreading rate to the molecular diameter and molecular weight of the internal gas involved.