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XII. Experiments on the Nature and Properties of the Pietra 
- fongaja, Lapis fungs er*, By P. A. Gappt. 
Stones ; in general are in Italy called Pietra; prune, 
which fometimes i is ufed to denote the harder fort of {tones, 
as well as ftones of a certain genus; for example, Pietra 
bigia, obfidiana, nephritica, &c.; and this is the eafe with 
the fo calied Pietra fongaja. 1.1. Ferber may be confidered 
as the firft perfon who gave a defcription of it in his letters 
from Italy. He has remarked alfo that a kind of it isin 
‘common ufe in the houfes of Naples and Rome; and that 
he faw another kind in the pofleffion of M. Fabbroni, at Flo- 
rence; the firft kind; which was found in the chalk-hills 
near Naples, confifted of white calcareous ftalaftites, and a 
number of fmall roots of vegetables; the latter was a hard- 
ened turf, dug up in the neighbourhood of. fome volcanic 
mountains. : 
A few years ago, M. Charles Sparre, chancellor of the 
academy, having been fo kind as to tranfmit tome a piece 
of pietra_fongaja, which he had brought with him from Italy. 
T analyfed it, and found the refult as follows :—It burns in 
an open fire, and emits the {mell of putrid vegetables. When 
burnt in a ftrong fire, the greater part of it becomes dark- 
grey afhes. A hundred parts of this hardened turf, loft 
about fifteen parts in weight. When fufed in a ftronger 
heat, it is converted into a black opaque flag, which, haw- 
ever, is difficult to be fufed. When a {mall part only of this 
earth is fufed with borax, the glafs acquires a dark-green co- 
lour. If a little water be poured over the calcined earth, 
- the water exhibits traces of diffolved pot-ath, -but the earth 
does not appear to have thereby fenfibly decreafed in-weight. 
One hundred parts of the earth, previoufly calcined in a cru- 
cible, being analyfed, were found to contain about 45 of 46 
of filiceous earth, 23 argil, 7 calcareous earth, 20 calx of 
iron, together with traces of magnefian earth and poteath. 
When the pietra fongaja is kept ina cellar, and moiftened 
* Muthroom-ftone, or mufhroom-beaving ftone. ° ' 
4& From Tranfac. Reyod Acad. Stockbolm for 1797. ; 
: with 
