atmosphere radiant matter, or fourth state of matter. He concluded that the remaining particles of the gas forming the radiant matter moved in straight lines over a great distance as compared with that moved through by molecules at the ordinary pressure. He called this distance the “mean free path.” If his theory is correct, this dark space is due to the fact that the molecules in motion at and near the cathode do not bombard each other and therefore do not produce the effect of light. When the motion is arrested by particles of gas themselves, within the bulb, then is light generated. The force propelling the particles from the positive pole was supposed to be less. In order to let the experiments speak for themselves, as much as possible, without being too much influenced by the opinion of the experimenter; the theory is only briefly alluded to as above, and will not be further applied in the presentation of his other experiments. In view of the radical discoveries of Lenard and Roentgen, after the installation of the Crookes phenomena, it has been the policy of the author to present all the experiments as facts for evidence in behalf of the general theories, which may be hereafter formulated independently of old theories. Therefore, the reader should bear in mind the teachings of the various experiments with the view of arriving at general principles and hypotheses. 54. RELATION OF VACUUM TO PHOSPHORESCENCE.—He started with such a high vacuum that he could not obtain any electrical discharge. § 25. There was, therefore, no phosphorescence in the glass tube, whatever. The caustic potash, which had been employed to absorb the last trace of moisture and carbonic acid gas, was slightly heated, and very gradually. Then it was noticed that a current began to pass and that the glass became green, and apparently on the inner surface. As the heat continued, the green passed gradually away and was replaced by striae, which first appeared to extend across the whole diameter of the glass tube (§ 40) which was a long cylindrical tube, and then became concentrated toward the axial line of the tube. Finally, the light consisted of a pencil of purple. § 10. When the source of heat was removed so that the moisture and carbonic acid gas could be absorbed again by the potash, the striae appeared, and then the other effects just named, only in the reversed order, until the tube acted like an infinite resistance. Phosphorescence is the correct word, because the light existed for a few seconds after cutting off the current.