PHOSPHORESCENCE PHOSPHORUS 463 rays of greater refrangibility than they them- selves, an action closely related to fluorescence. (See FLUORESCENCE.) It may be said in gen- eral that the color emitted by phosphorescent substances varies as they are insolated by light of different parts of the spectrum. Thus Can- ton's phosphorus may shine with a greenish light if excited by rays from one part, while with undecomposed light it appears yellow. Phosphorescent tubes have been made in Ger- many and France for several years, and their preparation was kept a secret ; but such tubes are now produced by several experimenters showing all the colors of the rainbow, and preparations may be made to imitate flowers and bright-colored insects, as well as land- scapes. The substances, after being prepared, may be stirred in a powdered state in melted paraffine, and any design may then be painted with them on glass plates. The paraffine pro- tects the powders from the action of moisture and prevents decomposition. The glass plates will glow for several hours after exposure to intense light with colors depending on the number and kinds of materials used. A more permanent mode of preservation is to seal the mixture in glass tubes or flat bottles. Green light may be produced by heating hyposul- phite of strontia 15 minutes over a Berzelius lamp, and then fusing in a blast lamp flame. Blue is obtained by heating precipitated sul- phate of strontia in a current of hydrogen gas, then over a Bunsen burner for 10 minutes, and lastly over a blast lamp for 15 or 20 min- utes. Should the light be yellowish, further heating with the blast lamp is required. Yel- low phosphorescence is obtained by fusing six parts of sulphate of baryta (heavy spar) with one part of charcoal over a blast lamp. At first no phosphorescence follows the fusion, but after 24 hours the substance acquires the power of emitting an orange-yellow light after exposure to the rays of the sun. A calcium or magnesium light may be employed in place of sunlight. Since it has been found that a great number of substances remain phosphorescent for a more or less appreciable space of time after exposure to light, it becomes a question whether all bodies whose particles are capable of being put into luminous vibrations by the action of the sun's rays do not give out light for some space of time afterward. M. Bec- querel invented an apparatus capable of mea- suring the duration of phosphorescence in dif- ferent bodies, which is called a phosphoro- scope. The apparatus is so contrived that the interval between the time of insolation and observation can be made as small as desired, and measured with the greatest precision. A stationary cylindrical box, A, of blackened metal, has an opening in the form of a circu- lar sector in each end, one being exactly oppo- site the other. One of these openings is shown at o. Within the box two circular screens, also of blackened metal, one at either end, are fixed to a common axis by which they are made to revolve. Each screen has four aper- tures, as shown at B, of the same shape as those in the cylinders, and at the same dis- tance from the centre. These apertures are not opposite each other, but alter- nate, so that a ray of light cannot pass through the machine. The substance whose phosphorescence is to be examined is placed in a stirrup suspended from the upper side of the cylinder, and may be raised or lowered by means of a milled head screw. The ap- paratus is placed in a window with the further side exposed to the sunlight, the machinery for turn- ing it being inside, and the room dark- ened. When the body is illuminated it can- not be seen by a per- son in the room, be- cause when the fur- ther screen uncovers the outer aperture of the external cylin- der the nearer screen closes the front aper- ture. On turning the screens one eighth of a revolution, the obstacle between the object and the eye of the observer is removed, and at the same time all source of illumination is cut off behind; and therefore if the object is now visible it must be by its own light, or in other words because of its phosphorescence. If it retains its phosphorescence for a longer time than it takes for the disks to make one eighth of a revolution, it will be visible ; but if it parts with it in less than that time, it will be invisible. If the revolutions are 160 a second, the length of time between the illumination and observation would be of T | 7 , or 0*00078 of a second. Quartz, sulphur, metals, and li- quids gave no appearance of phosphorescence. The uranium compounds presented the most beautiful appearance, remaining brightly lumi- nous 0-003 to 0'004 of a second after insolation. PHOSPHORUS (Gr. $Z>q, light, and tfpeiv, to carry), an elementary body, discovered by Brandt of Hamburg in 1669, in the solid resi- due left on evaporating urine, while attempt- ing to obtain a liquid capable of transmuting silver into gold. Kunckel, a German chemist, learned the source of the new substance and communicated the information to Kraft of Dresden, who went to Hamburg and paid 200 dollars for the details of the process. In the mean time Kunckel succeeded in preparing Phosphoroscope.
