fome Mineral Subjlances. i 
;/?#/ (kidney-formed cryftal). It is in the form of an 
uneven craft, with fmall protuberances; a part of it re- 
tains all the appearances of cryftal, but the reft is opaque, 
fomewhat of an afh colour, and is eafily powdered : the 
underpart retains marks of galena or potter’s lead ore, 
and in fome fpecimens the greateft part is coated with it. 
I took three drams of the opaque part of this cryftal 
finely powdered, and mixed it with an equal quantity of 
fixed alkaly of cream of tartar, and in order to facilitate 
the union, the mixture was made into a pafte with dif- 
tilled water, then dried and calcined for an hour as in my 
experiments on horn filver (fee Phil. Tranf. vol. LXVI. 
p. 604.). The mixture was then taken out of the phial 
powdered, and, by digeftion with diftilled water at three 
different times, deprived of its faline part : thefe three 
folutions were mixed together, faturated with diftilled 
vinegar, evaporated to drynefs and waflied well with 
rectified fpirit of wine. By this means %j. and gr. vi. of 
tartar of vitriol was obtained. The undiffol ved part, re- 
maining after digeftion with water, when dried weighed 
^ ij. ana gr. xxiv. ; this was mixed and digeftcd with pure 
acid of nitre, and effervefced ftrongly. After this di- 
geftion the undiffolved part here remaining, being well 
wafhed and dried, weighed ^ j. and gr. xv. which I take 
to be cryftal, for no faline matter was obtained by treating 
