PHOSPHORI. Bodies which emit light in the dark, after having been exposed to light, are called " solar phosphori." When we consider that bodies which have been exposed to heat continue to radiate heat for some time after being removed from the source of heat, and that heat and light are most probably undulations in the same ether, differing only in the length of the wave, the existence of solar phosphori cannot be considered a remarkable phenomenon ; on the contrary, the wonder is that so few bodies should exhibit the property possessed by them.
The first solar phosphorus on record was discovered about the year 1630 by Cascariolo, a shoemaker of Bologna, who found that calcined sulphate of baryta was luminous in the dark after having been exposed to sunshine, and that it continued luminous for some hours. The matter speedily assumed importance, and considerable quantities of calcined Bolognian spar (the native sulphate employed) were sold as an article of curiosity.
" Canton's phosphorus" is another example of the property. It is made by first calcining oyster shells in the open fire for half an hour ; then selecting the largest and whitest pieces, mixing them with about one-third their weight of flowers of sulphur, pressing the whole into a crucible with a closely luted cover, and keeping it at a red heat for an hour. The contents when cold may be turned out and the best pieces selected. They will be found to shine in the dark after having been exposed to sunshine.
Nitrate of lime fused at a dull red heat is also a solar phosphorus.
The blue and violet rays appear to be most effective in producing the phosphorescence. It is not found that solar phosphori generally emit light of the same colour as that to which they have been ex posed. There are phosphori from heat as well as from light. The native phosphate of lime found near Estremadura, in Spain, and also some varieties of fluor spar, particularly one called " chloro phane" are the most remarkable instances. These become luminous when slightly heated, or by fiiction.
Some animal substances are spontaneously phosphorescent. The flesh of the tench, carp, herring, and sole is luminous before putre faction commences. The property is rarely possessed by the flesh of quadrupeds, and has never been observed in that of birds. The phosphorescence of the sea is a beautiful phenomenon frequently observed, but one which has not yet been accounted for. It is probably due to the presence of phosphorescent animal matter.
The glow-worm and fire-fly are fiunilier instances of phosphores cence possessed by living animals.
Decayed wood and certain mosses have been known to exhibit phosphorescence, but the property is rare in the vegetable kingdom.
Some salts, (sulphate of potass for instance,) emit light during metallization.