of three microfarads each were connected in shunt with the break, and the capacity adjusted carefully until the break-spark was a minimum and gave a sharp cracking sound. Too much capacity will not necessarily increase the sparking, but it will diminish the inductive effect which is noticed immediately in the diminished intensity of the discharge. A powerful coil with a smoothly working rotary interrupter will be found a most satisfactory apparatus in experiments with Röntgen radiance.” § 106, 109, 114, 131, 137. 132. SALTS FLUORESCENCE BY X-RAYS. See also, _Elect. Rev._, N.Y., April 19, ’96, p. 165. Edison examined over 1800 chemicals to detect and compare their fluorescent powers if any, under the action of X-rays first transmitted through some opaque material such as thick cardboard. Of all these, calcic tungstate by measurement, fluoresced with six times the luminosity of barium platino cyanide, which was referred to in connection with Roentgen’s experiment. Other authorities agree as to its great sensitiveness. In making this comparison, it was assumed that the power of the X-rays varied inversely as the square of the distance from the discharge tube. Between the two above chemicals came strontic tungstate. Baric and plumbic tungstate scarcely fluoresced. Salicylate of ammonium crystals equalled the double cyanide of platinum and barium, and differed therefrom in that the fluorescence increased with the thickness of the layer of crystals up to 1/4 of an inch, showing great fluorescing power and low absorptivity. This experiment showed that the best fluorescent materials were not necessarily the salts of the heaviest metals, like platinum. It is assumed that the reader knows the difference between phosphorescence and fluorescence, but the dividing line is so difficult in some cases and the one not being distinguished from the other by experimenters, that the author has used the same words as the experimenters, although he admits that fluorescence is often meant where phosphorescence is stated, and _vice versa_. An anomaly presented itself as to rock salt, which although transparent to light yet powerfully absorbed X-rays and was strongly fluoresced thereby. Again, fluorite which is transparent to light, fluoresced strongly with the X-rays, and under their action became brighter and brighter and continued after cutting off the X-rays, the material therefore, being highly phosphorescent, the light enduring for several minutes. Upon watching the phosphorescence of fluorite, the same penetrated the plate very slowly to the depth of one-sixteenth of an inch, but beyond that depth there was complete darkness. The only other truly phosphorescent substance noticed was calcic tungstate, especially in thick layers, so that the shadow of the bones of the hand remained thereon for a minute or two upon cutting out the discharge tube from the circuit. Some chemicals, within a dark box and _very close_ to the discharge tube,