fame Mineral Subjlances. 25 
which is found with lava, and is the product of a vol- 
cano. I had none of it to try, but judge it, from its ap- 
pearance, to be of a different nature from that found in 
mines. 
Allum is commonly obtained from llate, which for 
that purpofe is calcined for a confiderable time. I know 
no fubftance fo replete with it as the Iridi date, lapis Hy- 
bernicus of the druggifts ; for this, without any calcina- 
tion, affords allum. 
Allum is likewife obtained from the earth or clay of 
Solfatara, and from the red date found near Saarbruck ; 
but thefe have been expofed, no one can tell how long, to 
the heat of volcanos. Iron always accompanies allum, 
and thence the ufe of fome alkaly, by which means the 
red chalk ( rubrica fabrilis ) is obtained. 
Of jafper. 
Many of the jafpers, fo called, owe their origin to 
cryftal or quartz coloured with iron; perhaps in a few 
inftances to copper. Thefe, I dare fay, treated as the for- 
mer, would produce no allum ; but I have only tried one 
of the Oberftein jafpers, and this had vidble marks of 
cryftal. Others of the jafpers, and thefe I call the true 
ones, are formed by clay and afford allum ; fuch is the 
Vol. LXIX. E Saxon 
