The electrical discharge in the Crooke’s tube is many-sided in its manifestations. Its energy seems all-pervading in the room where it occurs. Before the discharge passes through the rarefied space in the tube its energy manifests itself by a crackling spark, a miniature lightning discharge. This spark, five or six inches in length, can send out magnetic waves which extend far beyond the narrow limits of the room. They can be detected, by the methods of wireless telegraphy, fifty miles. When the same amount of energy is developed in a Crooke’s tube the magnetic waves hardly pass beyond the walls of the room, and the phenomenon of phosphorescence and fluorescence and the strange molecular effects outside the Crooke’s tube spring into prominence. The crackling spark outside the tube is far-reaching in its effect, yet it shows no signs of the X rays, its light can not penetrate the human body, it excites only a feeble phosphorescence at a distance of even two or three feet, while the same energy excited in the Crooke’s tube can cause luminescence at a distance of twenty feet. The crackling spark, however, can be seen much farther than the light of the Crooke’s tube, and it can also impress a photographic plate at much greater distance. The following experiments will illustrate the different manifestations of energy of which an electrical discharge is capable. I produced an electrical spark about six inches in length and exposed a photographic plate for six seconds, at a distance of two, ten, and twenty feet, to its light. A thin strip of tin, with a circular hole cut in it, served as a shutter. The sensitive plate was thus protected, except in front of this aperture. The images exhibit the decrease in light with the increase of distance. Another portion of the sensitive plate was exposed in the same manner during the same length of time to the light of a Crooke’s tube which was excited by this same spark. No image was obtained at a distance of ten feet, and barely one at three feet. The spark in air, therefore, was far more energetic photographically than the X rays, but it could not penetrate solid materials. This property was given to it by its passage through rarefied space. I then covered a screen with a phosphorescent substance, and exposed it to the spark in air. The phosphorescent light could barely be detected at a distance of three feet, while with a spark in rarefied air it could be seen at a distance of twenty feet.