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Species of 'Pyrites j infomuch that, unlefs it werd 
taken in the Hand, whereby their fpecific Gravities 
may be determined, it might pafs for one of that 
Family ; altho’ this Stone is by much the moft fpe- 
cifically heavy I ever faw of this fort. 
I imagine, that, a conftderable Time before the 
Hoife died, by fome Accident this Stone received a 
great Blow 5 for there appears to have been a Piece 
broken out ; and there arc two large Cracks not yet 
fill’d up; near which terminate feveral concentric 
Circles. This Stone feems to involve a fmaller one, 
altho’no-where perfectly ftparated from it 5 but the out- 
ward is by much the hardeft. In the Centre are two 
Holes, in which maybe fecn feveral Hairs of the Horfe 5 
but I have not been able to find any other extra- 
neous Body, upon which thefe Calculi are ufually 
formed. 
Having, from fawing the Stone, a Quantity of its 
Powder, I was induced to an Inquiry into its con- 
ftituent Parts by way of Analyfis. 
I firfi let fall two fmall Pieces of this Scone into 
Water al moft boiling : They immediately funk, but 
arofe again, and continued alternately riling and fink- 
ing a conftderable time. This was occafioncd by the 
Quantity of Air-Bubbles, which the Heat rarefy ’d j 
but the Air was detain’d by the Mucus , which feemed 
to conned the Particles of the Stone together j and 
which, tho’ diluted by the hot Water, was tenacious 
enough to form Bubbles of Size fufficient to buoy 
up the Pieces of Stone ; the Rarefaction growing 
greater, the Bubbles burft, and the Scone fell to the 
Bottom; but arofe again, in like manner, at the 
Expulficn of more Ain The learned Dr. Hales \ ike- 
