182 ESSAYS AND OBSERVATIONS 
thofe that are converted into a perfect 
quick-lime in a {trong fire, fuch as lime- 
f{tone, marble, chalk, thofe {pars and marles 
which effervefce with aqua fortis, all ani- 
mal fhells, and the bodies called /ithophyta, 
All of thefe, by being joined with acids, 
yield a fet of compounds which are very 
different from thofe we have juft now de- 
feribed. Thus, if a fmall quantity of any 
calcarious matter be reduced to a fine pow- 
der and-thrown into fpirit of vitriol, it is 
attacked by this acid with a brifk effervef- 
cence; but little or no diffolution enfues. 
It abforbs the acid, and remains united 
with it in the form of a white powder, 
at the bottom of the veflel, while the li- 
quor has hardly any tafte, and fhews only 
a very light cloud upon the addition of 
an alkali*. 
THE 
* Mr Margraaf has lately demoniftrated, by a fet of 
curious and accurate experiments, that this powder is of 
the nature, and pofleffes the properties, of the gypfeous 
cr felenitic fubftances, That fuch fubftances can be re- 
folved into vitriolic acid and calcarious earth, and can 
be again compofed by joining thefe two ingredients to- 
gether. Mem de l’Acad. de Berlin, an. 1750, p. 144. 
