SCIENCE. 



355 



the world emphatically, that the long sought for 

 " perpetual light " had at last been found, while an- 

 other enthusiastic novice wrote a work upon the 

 Phosphorus mirabilis and its marvellous brilliancy. 

 Here again the future did not justify all the hopes which 

 might have been expected. This substance, however, 

 which still goes by the name of phosphorus has become 

 one of the necessities of our age. 



Phosphorus gradually entered the scientific period. In 

 1768 an English chemist, Canton, obtained a new kind 

 by calcinating oyster shells with sulphur, and it was 

 finally discovered that the best absorbents of light were 

 combinations of sulphur, calcium, haryum and stron- 

 tium. However, other metallic sulphurcts and various 

 substances are equally capable of making in the dark 

 what is called solar, magnetic or electric light. The 

 method of preparation, of course, has considerable in- 

 fluence and lights of divers colors can be obtained ac- 

 cording to the process employed. By calcinating sul- 

 phates with organic substances, or carbonates with sul- 

 phur, a very brilliant phosphorus can be obtained, con- 

 sisting principally of baryta, another of lime, less lumin- 

 ous and a third of strontium, which gives forth a very 

 feeble light. Sulphate of baryta gives a phosphorescent 

 product of an orange color. When the sulphate is pre- 

 pared artificially the light is greenish. 



Later, Ozarm obtained other luminous bodies by cal- 

 cinating lime with sulphate of arsenic or sulphate of anti- 

 mony, while another chemist, Bach, by heating sulphur 

 with calcinated oyster shells which had probably been 

 washed with a solution of ammoniac and realgar, pro- 

 cured a phosphorus so brilliant that its light was even 

 visible during the day. 



It is by this means, or others which are similar, that 

 the luminous flowers are prepared which lately have ap- 

 peared to such an extent. They are covered with some 

 phosphorescent substance which makes them glimmer in 

 the dark with a beautiful bluish light. The luminous 

 matter is pulverized and applied to the object by means 

 of a varnish or anything else that will stick. By employ- 

 ing phosphorus of different colors very pretty effects can 

 be produced, bouquets of all shades, glittering butterfles, 

 luminous inscriptions, etc. But the most interesting of 

 all is undoubtedly luminous photography, which is made 

 by placing a paper covered with phosphorescent powder 

 behind the glass negative of a photograph. Heat brings 

 out the luminous qualities as well as light, and very pe- 

 culiar and beautiful effects can be obtained by writing 

 upon such a paper as has just b en described with a 

 pointed piec of heated metal. 



Unfortunately these interesting amusements are rot 

 eligible as regards trade, for as soon as it is exposed to 

 the air sulphuric luminous matter gradually loses its 

 properties and acquires the disagreeable odor of spoiled 

 eggs, while the object by the end of a week or two is 

 not phosphorescent at all. On the other hand, it can be 

 very well preset ved by putting it into air-tight glass 

 tubes, and phosphorus of all colors thus prepared can 

 be had from Geissler's establishment in Bonn. It has 

 been proposed to make inscriptions of these tubes for the 

 night bells of hotels, physicians' houses and druggists' 

 shops, the daylight being sufficient to make them very 

 luminous at night. Another idea, conceived by Gustave 

 Ullig, is to make the faces of watches and clocks phos- 

 phorescent, as the glass covering them would be a pro- 

 tec ion against destruction. 



As to the physical explanation of phosphorescence, it 

 was thought for a long time that the light was composed 

 of little eddies or whirlpools of subtile matter, and that 

 sunlight became condensed and accumulated in them. 

 Later, when it was known that light is only a vibratory 

 movement, and that the phosphorus on the end of 

 matches only burns because it is united with the oxide 

 in the air, it was thought that in all the old phosphor- 

 escent substances the light was produced alone under 



the influence of a slight oxidation. This explanation, 

 however, is false, and only during the last century was 

 the true one made known by a celebrated German phy- 

 sician named Euler. 



It is generally believed that the planets, the tops of 

 mountains, and all celestial bodies, are visible, simply 

 because they reflect the rays of the sun. This is also 

 false. Brilliant surfaces alone, more or less, reflect light, 

 others absorb it on the contrary, and cause vibration 

 just as a musical sound makes all the objects which it 

 strikes vibrate. Certain surfaces, however, can only re- 

 produce certain vibrations (blue or red for example) of 

 solar light, which is composed of the vibrations of the 

 seven prismatic colors, and when these vibrations are re- 

 peated in our eye, the surfaces appear to us blue or red, 

 as the case may be. 



In the same way that consecutive vibrations can be 

 determined after sound, so phosphorescence succeeds 

 the action of l'ght. Euler affirmed ihat the greater 

 number of bodies would present these luminous vibra- 

 tions if they were observed immediately after they had 

 been exposed to the sun, and if a continued sitting in the 

 dirk had rendered the eyes of the observer very sensible. 

 Tne French physician, Becquerel, constructed an instru- 

 ment about twenty yeats ago called the phosphoroscope, 

 by means of which he demonstrated that most sub- 

 stances, paper, stone, oyster shells, etc., shone for a 

 short time after being exposed to the light, that is to say, 

 a second or the fraction of a second, and that solar phos- 

 phorus was only distinguishable from other bodies by 

 the persistence of this property. But whether this as- 

 sertion be true, generally speaking, or not, the subject 

 itself is not by any means simple, and there are a mass 

 of circumstances of which we must take account. 



Modern physics teach us that a number of bodies, nota- 

 bly colored organic matter and some metallic combina- 

 tions, become phosphorescent, but only when they are 

 lighted. This sounds like a paradox, but facts can prove 

 the assertion. There are certain substances, both liquid 

 and solid, which by reflected light appear to have another 

 c lor than the one transmitted. A peculiar emission of 

 rays can also be observed upon the surface. Petroleum, 

 solutions of sulphate of quinine, decoctions of Indian 

 bark, etc., emit bluish rays ; the etherized extract of green 

 leaves, blood red rays ; uranium glass, which is pale green 

 and used principally in the manufacture of Rhine wine 

 glasses, emits reddish yellow rays. If any of these 

 dichroic subtances are selected and placed in a dark 

 room lighted only by an electric current traversing a glass 

 tube, they will shine brilliantly, each one in its particular 

 color, certainly with more splendor than the electric light, 

 and yet only while the latter illumines them. How is this 

 curious phenomenon to be explained ? How can a feeble 

 light produce such a brilliant one ? 



It has been said above that white light is composed of 

 seven colors, or, more properly speaking, of an infinite 

 number of colors, which after their dispersion from the 

 prism separate one from the other and form a long band. 

 The red rays are those which vibrate the slowest, and the 

 violets those which vibrate the most rapidly. But just 

 as there are in addition to the red rays others which 

 vibrate slower still and are manifested not as luminous 

 rays, but calorific ones, so there are besides the violets, 

 ultra violet rays which vibrate so quickly that we cannot 

 directly perceive them, although they are known by their 

 energetic chemical action. This is notably the case in 

 photography, and for this reason they are termed chemi- 

 cal rays, or invisible light. A pale, electric light, is a 

 peculiarity of these rays, and the latter give to certain 

 bodies that remarkable dichroic radiance which has been 

 called fluorescence, because it was observed for the first 

 time in fluor-spath. However, if on one side these rays 

 produce a light which cannot be perceived by our retina 

 owing to the extreme rapidity of their vibrations, on the 

 other, the bodies thus illuminated should be able to 



