feme Mineral Sub/lances. 17 
this experiment it appears, that givxcviii. of vitriol was 
produced by gr. cxiv. of pure gypfum, for the gr. vi. of 
clayey matter muft be deducted. 
The like quantity of Dcrbyfhire cauk, treated in the 
fame manner, produced gr. lix. of tartar of vitriol, and 
the clayey matter, which exactly refembled the former, 
weighed gr. xxiii. 
The ftellated fpar from the ifland of Sheppy, fub- 
mitted in the fame proportion to the fame trials as the 
foregoing, produced gr. lviii. of tartar of vitriol, and gr. 
xxii. of clayey matter, which was whiter than in the 
former. 
Woodward’s erica formed fpar in the like proportion 
and manner as the former, afforded gr. xxxviii. of tartar 
of vitriol, and gr. xlii. of clayey matter. 
To fhew that the calcareous earth in the gypfum con- 
tained a greater portion of acid of vitriol than that in the 
felenitical fpars, I reafoned thus, having previoufly fub- 
tradted the clayey matter of each. 
If 1 14 grains of plafter (the 6 grains of clay being 
deducted) afford 98 grains of tartar of vitriol, how much 
fhould 97 grains of the gypfeous matter contained in 
cauk afford (for the clayey fubftance muft alfo here be 
fubtradted). From this rule it fhould produce 8311- 
grains of tartar of vitriol, and neverthelefs the quantity 
Vol. LXIX. „ D 
was 
