122 BOLOGNA PHOSPHORUS. 
become capable of imbibing so much light on exposure, 
for a little while, to the light of the sun, or even to the 
flame of a candle, that it will afterwards shine in the 
dark for an interval of froba eight to fifteen minutes, 
like a glowing coal, but without any sensible heat. The 
light which it emits is sufficient to read by, provided the 
letters be placed near it. If well prepared, the stone 
will retain this extraordinary property for five or six 
years. 
The preparation of it is thus conducted. Pieces of 
sulphat of barytes are made red hot, for a few minutes, 
in a covered crucible placed in the middle of a fire, and 
then left to cool. When cool, they are pounded in a 
stone mortar, and sifted. The powder thus formed is 
made into a paste with a little gum arabic, and divided 
into long cakes, or cylinders, each about a quarter of 
an inch thick. These pieces are dried in a moderate 
heat, and then, by degrees, are exposed to a more 
violent heat, among charcoal, in a \\Hnd furnace. As 
soon as the coals of the furnace are half consumed, it 
must be filled a second time, and the phosphorus must 
be left undisturbed. When the coals are quite consumed 
the ashes must be carefully blown off with a pair of 
bellows, and the phosphorus will be found at the bottom 
of the grate. 
