PROSPHORES'CENCE. Strictly speaking, the term is applied to the phenomenon, exhibited by certain bodies, of retnaining luminous in the dark for some time after being exposed to a strong light. In this sense, it is strictly analogous to, perhaps we-should say, identical with, the heating of bodies by exposure to light or radiant heat. They absorb part of the energy of the vibrations which fall on them; it becomes motion of their particles; and is again radiated from them as light or heat. Certain preparations. such as Canton's phosphorus (q.v.), indurated limestone, etc., possess this true phos• phorescence in a very high degree. With the great majority of phosphorescent bodies, however, the duration of the phenomenon is very short, rarely more than a small frac tion of a second. Becquerel, who has recently studied this phenomenon with great care, has invented a very ingenious instrument for the purpose, called a pleoxpleoroscope. The body to be tried is placed in a small drum, which has an opening at each end. In this drum there revolve two disks, mounted on the same axle, and pierced symmetrically will& the same number of holes. They are so adjusted that when a hole in one disk is opposite to the hole in the corresponding end of the drum, the second disk closes the hole at its end of the drum, mid vice versa'. Light is admitted by one of the holes in the drum, so as to fall on the object, and it is examined through the other hole. It is obvi ous that when the disks are made to revolve, the object is alternately exposed to light, and presented to the eye. By a train of multiplying wheels, these alternations may be made to succeed each other as rapidly as the observer pleases, and thus the object is pre seined in the dark to his eye as soon after its exposure to light as may be desired. Almost all bodies are found to be phosphorescent; for instance, some kinds of pink rubies, when exposed to sunshine in this appe'ratus, appear to glow like live coals in the dark. The phenomenon is, in fact, precisely that which was observed by Brewster and Herschel in quinine and certain crystals of fluor-spar, and thence called fluorescence. Stokes was the first to give the true explanation of these facts, and he showed it to depend upon the change of refrangibility (i.e., color) which light suffers on being absorbed and then radiated by the fluorescent substance. The green coloring-matter of
leaves, a decoction of the bark of the horse chestnut, and the common canary glass (col. ored with, oxide of uranium), are bodies which exhibit this phenomenon very well. Perhaps the most striking method of studying the phenomenon is to receive in a dark ened room the solar spectrum (q.v.) on a sheet of white paper; and to pass over the col ored spaces it brush dipped in a solution of sulphate of quinine with sulphuric acid. No change is produced on the less refrangible rays, but in the blue and indigo spaces it strange change of color is at once apparent where the liquid has been spread. This appears more strongly in the violet, and vividly in the spaces beyond the violet, where lays fall which excite no luminous sensation in the eye. By this experiment, the visible length of the spectrum may easily be doubled. By using the electric light, which is peculiarlyrich in these highly refrangible rays, a prism of quartz, which allows them to pass very freely, and various fluorescent substances, Stokes has obtained spectra six or eight times as long-as those otherwise visible. The characteristic of all these rays is that they are less refrangible than those from which they are produced. The entire phenom enon is identical in principle with Leslie's photometer, in which light was measured when changed into heat by absorption, in the colored glass of which one of the bulbs of his differential thermometer was formed.
Ordinary phosphorus (from which the phenomenon took its name) becomes luminous in the dark by slight friction; whence the common trick of drawing self-luminous fig ures on doers and walls with a stick of phosphorus, or an ordinary lucifer-match. A similar appearance is presented by putrescent animal matter, such as decaying fish, etc.; but these are effects of slow combustion, or chemical combination, and are not properly classed among the phenomena of phosphorescence. See LUMINOSITY OF BEINGS.
This substance affords an excellent example of allotropy; that is to say, it may he made to occur under different forms presenting different properties. See ALLOTROPY.
Ordinary phosphorus and the red variety are the only important forms. We shall speak of them as phosphorus and red phosphorus respectively.