58. SHADOW CAST WITHIN THE DISCHARGE TUBE. This is illustrated in Fig. 11, p. 17, where there is a negative polar disk at the small end of the egg shaped tube, and a cross near the large end, the same forming the positive pole. The cross is made of aluminum. There was a novel action, however, discovered in addition to the mere casting of a shadow. The glass which had become phosphorescent except within the shadow, became after a while, less phosphorescent. Its property to phosphoresce became less as proved by removing the cross, which was arranged to fall down upon tipping the bulb. Immediately, the part which was within the shadow became brighter than the rest of the glass, thereby reversing the appearances, by making a luminous picture of the cross upon only partially phosphorescent glass. A remarkable feature is that the glass never recovered its first exhibited power of phosphorescence, neither did this power entirely become nothing, however many times the tube was employed. Was the deposit of metal from the cathode the cause? 58_a_. MECHANICAL MOTION PRODUCED BY RADIATIONS FROM THE NEGATIVE POLE. It occured to Crookes that the radiations from the cathode might perhaps cause a wheel to turn around. He therefore had a minute wheel made by Mr. Gimingham, like an undershot water wheel, and its axle rested on two rails of glass, so that it might roll along from one end of the tube to the other. The vanes were exactly opposite to the plane surface of the cathode. The molecular stream or radiations, or whatever they may be, possibly vibrations, from the cathode, were so powerful mechanically that the wheel was caused to run up hill, the tube being inclined very slightly. On the principle that action and reaction are equal, he built another device in which the negative electrode was movable, and he observed that when the current was on, the negative electrode moved slightly. Upon these principles he built the well-known Crookes radiometer in which the vanes rotated by reaction of the radiations. The vanes in this form of radiometer were made of aluminum, and a cup of hard steel served as the bearing, Fig. 12, p. 17. One side of each disk was coated with a thin scale of mica. The aluminum disks formed the cathode, while the anode was located at the top. The operation consisted in connecting the terminals as stated, so that the vanes were the negative poles and it was observed that the little wheel rotated. The vacuum was not as high as that for obtaining phosphorescence. With a low vacuum, an envelope of violet light existed near the surface of the aluminum vanes. Effects were carefully studied by maintaining connection with the pump. At the pressure of .5 mm. there was a dark cylinder opposite the aluminum extending to the glass, and this was the pressure at which the vanes began to rotate. The dark spaces opposite each vane became larger and larger in width, until they appeared to be opposed or