We were standing by the window at another time. Paul was engaged in polishing a number of lenses, which were ranged along the sill. Suddenly, after a pause in the conversation, he said, “Oh! I’ve dropped a lens. Stick your head out, old man, and see where it went to.” Out I started to thrust my head, but a sharp blow on the forehead caused me to recoil. I rubbed my bruised brow and gazed with reproachful inquiry at Paul, who was laughing in gleeful, boyish fashion. “Well?” he said. “Well?” I echoed. “Why don’t you investigate?” he demanded. And investigate I did. Before thrusting out my head, my senses, automatically active, had told me there was nothing there, that nothing intervened between me and out-of-doors, that the aperture of the window opening was utterly empty. I stretched forth my hand and felt a hard object, smooth and cool and flat, which my touch, out of its experience, told me to be glass. I looked again, but could see positively nothing. “White quartzose sand,” Paul rattled off, “sodic carbonate, slaked lime, cutlet, manganese peroxide—there you have it, the finest French plate glass, made by the great St. Gobain Company, who made the finest plate glass in the world, and this is the finest piece they ever made. It cost a king’s ransom. But look at it! You can’t see it. You don’t know it’s there till you run your head against it. “Eh, old boy! That’s merely an object-lesson—certain elements, in themselves opaque, yet so compounded as to give a resultant body which is transparent. But that is a matter of inorganic chemistry, you say. Very true. But I dare to assert, standing here on my two feet, that in the organic I can duplicate whatever occurs in the inorganic. “Here!” He held a test-tube between me and the light, and I noted the cloudy or muddy liquid it contained. He emptied the contents of another test-tube into it, and almost instantly it became clear and sparkling. “Or here!” With quick, nervous movements among his array of test-tubes, he turned a white solution to a wine color, and a light yellow solution to a dark brown. He dropped a piece of litmus paper into an acid, when it changed instantly to red, and on floating it in an alkali it turned as quickly to blue.