1 6 Mr. woulff.’s Experiments on 
Two fpars from the Hartz, compofed of flakes and 
cryftallized. 
Ditto from Saxony. 
Striated fpar from Scotland, brought here by Mr. 
GOAN. 
Spar intermixed with cinnabar and ochre of iron from 
Obermofchel in the dutchy of Deux Fonts. 
Red ochry comp aft fpar from Derbyfhire. 
I fhall give a few of my experiments on tliefe fub- 
ftances compared with the common Paris montmartre 
plafter ftone; all thefe were deprived of their humidity 
before they were fubmitted to trial. 
Two drams of the Paris plafter ftone, treated as in the 
former experiment, with an equal quantity of alkaly of 
tartar, produced g jfC and gr. viii. of tartar of vitriol '. 
The infoluble part remaining after being deprived of 
its faline part by water, was digefted with diftilled vine- 
gar, which effervefced and diflolved the whole of it, ex- 
cept a fmall portion, which, when wafhed and dried, 
weighed gr. vi. and had the appearance of tobacco-pipe 
clay. Had I taken a pure white tranfparent gypfum, I 
believe that no fediment would have been left. From 
( a ) The tartar of vitriol, of all the trials I make mention of in this paper, 
was diffolved in diftilled water, and mixed with a folution of calcareous earth 
in acid of fait, which caufed a copious felenitical precipitation, a clear demon- 
ftratioh of the exigence of the acid of vitriol. 
this 
