Experiment 3. — Rub the smooth handle of a fine awl over a piece of fine wire gauze, and the gauze seems to present a continuous sur face. Perhaps it is the fault of the instrument in your hand, and not the fault of the gauze. Rub the point of the awl over the gauze, and you soon find openings between the metal threads. But the openings are there, whether you can feel them or not. Experiment 4. — Rub the point of a fine sewing needle over the surface of a window pane. The glass seems to be continuous in its structure, and the needle cannot get through. Perhaps it is the fault of the instrument you are using. Try one more delicate. Let a ray of light fall upon the glass, and it easily finds a passageway between the solid molecules. Rays of light are often used by scientific men as instruments for their work.