5l6 PHILOSOPHICAL TRANSACTIONS. [aNNO 1681. 



cular; these last are often the best, as being more shining and transparent. 

 They are usually as large as an orange; and though Licetus says there never 

 was any greater than that in Aldrovandus's museum, weighing 2ilb, yet the 

 author has had them of 5 ib-* 



The colour is various, as ash, sky, rusty, yellow, earthy and white; but the 

 best for this use are sky-coloured and white. When it is prepared, it has in it 

 an efflorescence, in which lies the greatest part of its virtue of receiving light, 

 which it does most from the sun, but less degrees from the day, the moon, 

 .or a flame. Though this light in the dark appears like a coal [gleed], it is not 

 sufficient to read by, unless applied close to the word : it retains not its light 

 long, nor its virtue above 5 or 6 years. The best way of seeing it is in a dark 

 room, after the eye has been there kept for some time, and the stone be brought 

 in immediately from being exposed to the sun. • 



For preparing it, make a cylindrical furnace of iron or copper plates, about 

 7 inches in diameter, and about 7 inches high. Line the inside of it with 

 strong lute, that the hollow of it may be about 6 inches over, kept from slip- 

 ping down by the lower edge of the plate turned in. Make at the top 4 

 notches, 2^ inches high, and 1-^ inch broad. To this make such another 

 cylindrical part of the same diameter, but a little higher, making 2 ash holes, 

 or air holes, opposite to each other at the bottom, large enough to put 

 in one's hand; line this also as the other with good lute, and make a bot- 

 tom also of lute to reflect the heat.. Make a cover in the same manner, 

 and line it with lute; then fit a grate of iron wire, which may sustain the 

 coals, and freely admit the air; place this between the lower and middle 

 parts, and lay upon it pieces of lighted charcoal, and upon those some others, 

 well charred but not lighted. When you have made this bed of charcoal, as 

 high as the notches, having made powder of some other of these stones beat 

 in a mortar, and searced, dip the stones to be calcined in good strong aquavita^, 

 and roll and cover them with that powder, then lay them close together on the 

 bed of charcoal, and make another bed of the like charcoal over them to the 

 top of the furnace, which then cover with the round close head. Let the coals 

 burn till all are consumed, and the remaining stones are cold ; then take them 

 out, and taking oflT the crust from them, wrap them up in silk, and keep them 

 in a close box till you use them. 



If you would cover any figure with it, beat the crust you took off into a fine 



* The so called Bolognian stoue is a sulphate of barytes. There are generally, however, Aggre- 

 gated with it otlier earthy substancesj besides a portion of iron. 



