( ) 
lid Body Is Immerged, under the Receiver of an Air- 
Pump, and to extract the Air out of the Body by that 
means ; which will be more eafily and exaddy done, 
if the Water be firft heated over the Fire. And where 
die Conveniency of an Air-Pump cannot be had, the 
fame Thing may be done almoft as well, by letting 
the folid Body continue fome Time in boiling Water 
over the Fire. 
But no folid Body muft ever be put into hot Water, 
that will in any meafure .diflolve, or give a Tindure to 
the Water. 
One Inftance of the Negled of this Caution, .may 
be feen in the Accounts we have of the Specifick Gra- 
vity of the Stones taken out of Human Bladders, 
which have been commonly found to be but about 
one half, and fome of them have been no more than a 
fourth Part heavier than an equal Bulk of Water. 
From this it has been too haftily concluded, that thefe 
Stones are very improperly call'd by that Name, as 
not at all approaching to the Specifick Gravity of 
even the lighted real Stones, that we have any Ac- 
count of. 
Whereas it is mucli more reafonable to fuppdfe, 
that thofe Stones, which have been fotrn<f to be fo 
light, were fuch as had been a considerable Time taken 
out of the Bladder, and confequently had loft much 
of their Weight by the Evaporation of the Urine, 
with which they had at firft been faturated, and that 
they had afterwards been tried without the Caution 
above- mention'd. I would therefore beg Leave to re- 
commend it to thofe, who fhall examine the Specifick 
Gravity of the Human Calculus , that they will either 
try the Experiment upon Stones frefh taken out of 
ttbe Bladder, or dfe that they will be pleas’d to ufetbe 
■x above- 
