22 Mr. woulfe’s Experiments on 
good cryftallized allum^. The fourth calcination af- 
forded no allum, and what remained, after being dried 
and wafhed, weighed gr. lv. What this matter is I have 
not yet tried; perhaps it may be of the quartzy kind. 
Hence it appears, that this clay contains half its weight 
of earth of allum, which, by its union with the acid of 
vitriol and the water that enters into its cryftallization, 
produces better than four times its weight of allum, and 
therefore this clay treated with acid of vitriol affords 
more than double its weight of allum. 
Of Feld fpar. 
The honourable Mr. greville, a member of the 
Royal Society, and remarkable for his tafte and fkill in 
natural hiftory, as well as for his judicious remarks on 
the nature, growth, and formation of minerals, has in 
his travels made fome very intereifing obfervations on 
the formation of Feld fpar; the fpecimens which he col- 
le£ted on the fpot fhew evidently the change of clay into 
this fpar, and alfo the different gradations of the change. 
(d) The celebrated Mr. margraf, to whom the difcovery of making allum 
with argilla and acid of vitriol is due, could not obtain the allum in a cryftal- 
lized ftate without the addition of fome fixed alkaly. His miftake was owing to 
theexcefs of acid of vitriol, which the alluminous earth of the clay retained for 
want of fufficient heat, and which he corrected by faturating it with an alkaly. 
The 
