on the Terra Ponderofa, &c. 
GE N ERA 7 O BS BUR WV) Ay Tei O Re se 
The terra ponderofa feems to claim.a place betwixt the earths — 
and the metallic calces. Like the former, it cannot be made 
to aflume a metallic form; but, like the latter, it may be pre- 
cipitated from an acid, by means of phlogifticated alkaly. In 
many of its properties it much refembles the calx of lead; 
and. in others, the common calcareous earth, but full feems 
fufficiently different from that to conftitute a new genus, as will 
appear from a little attention to. the following circumftances. 
Terra ponderofa,,. ‘Heura caledres, "14 
When diflolved in water, preci- | Diffolved in water, does not 
pitates. upon the addition of | . precipitate upon the addi- 
the {malleft portion of vi- tion of vitriolic acid.. 
triolic acid. 
Its gypfum, therefore, is in- | Its. gypfum, therefore, is fo- 
foluble. ) waduble,. 
With the nitrous and marine | With nitrous and marine acids 
acid, forms cryftals which | forms falts fo deliquefcent 
do not deliquefce. 7 that they cannot. be kept in 
: | a cryftallized. form. 
Decompofes vitriolic falts v7a | Does not decompofe vitriolic 
humida. falts. 
Tt has been: called terra ponderofa, or heavy earth, upon 
account of the great fpecific gravity of its gypfum; its fpar 
is likewife heavy enough to countenance fuch an appellation ; 
but the earth itfelf does not appear to be a heavy fubftance, 
and I imagine the great weight of its compounds with the 
vitriolic and atrial acids 1s owing to the abfence of water. 
Birmingham, Nov. 1783.. 
RASA 
