be stated incidentally, that when the magnet was moved to and fro, the pencils of light waved back and forth. In the modified form of construction over that shown in Fig. 15, p. 17, he caused a wheel to rotate that was located in the high vacuum. The vanes of the wheel were so located that the faces of the same were perpendicular to the direction of the pencil of the rays radiating from the cathode. When the magnet deflected the rays, the wheel ceased rotation. 60. MUTUAL REPULSION OF CATHODE RAYS.—If the little mica screen, as shown in Fig. 16, p. 17 has two holes, and if there are two cathodes instead of one, there will also be two pencils of light. He performed an experiment involving the latter modification, and the result was something that could not have been predicted. The two pencils, as displayed by the long fluorescent screen, repelled each other like molecules similarly electrified. The white pencils, it will be noticed, were repelled from each other and showed their condition when both of the negative poles were in circuit. The black pencils show the location of both of the pencils when only one pole is in circuit at a time, the direction being perpendicular to the plane of the cathode disc (§ 57) at end. 61. HEATING AND LIGHTING POWER OF CATHODE RAYS. HEAT OF PHOSPHORESCENT SPOT.—By making the cathode concave as in Fig. 10, p. 17, and so locating it that the focus of the cathode rays falls upon some substance, the latter becomes very hot. In this way Crookes melted wax on the outside of the bulb at the phosphorescent spot. Further than this, the heat was so great that it cracked the glass without at first injuring the vacuum; next the glass at this point softened, and the air, by its pressure, rushed into the bulb, forcing a hole through the soft part. He performed an experiment also which illustrated the intensity of the heat when the rays were brought to a focus. He used an unusually large electrode like a concave mirror, and in the focus, which was near the centre of the bulb, he supported a small piece of iridio-platinum. At first, with a moderately low E. M. F., the metal was made white hot. When a magnet was caused to approach, the rays were drawn to one side, § 59, and the little piece of metal cooled. He then put in all the coils of an inductorium, and allowed the metal not only to become white hot, but to become so heated that it melted. How little did Prof. Crookes know about the most important phenomena associated with his experiment. Although he was so exceedingly enthusiastic and ingenious in planning his experiments, and in reasoning, yet it seems almost mysterious that he should have been subjected to what have become known as X-rays, which passed into his body, and would have photographed portions of his skeleton, and which would have performed outside of the tube many of the acts that were noticed within. Seventeen years elapsed between the time