The sodium vapour etching experiments of Andrade & Tsien (1937) were repeated with chemical Pyrex glass. By employing a ball indenter in conjunction with the vapour etching, it was possible to determine with greater accuracy the real nature of the surface cracks that were developed. Thereby, it was established that the surface stressing action of the sodium vapour not only dilates the existing cracks, but also forms additional extensive networks. Furthermore, the ball indenter is capable of distinguishing the real from the artificial cracks. Glasses other than chemical Pyrex were found resistant to etching.