L *53 ] 
lighter than water, or io^ lighter than common 
air. 
This method of finding the denfity of factitious air 
is very convenient and fufliciently accurate, where the 
denfity of the air to be tried is not much lefs than that 
of common air, but cannot be much depended on in 
the prefent cafe, both on account of the uncertainty in. 
the denfity of common air, and becaufe we cannot be 
certain but what fome common air might be mixed 
with the inflammable air in the bladder, notwith- 
standing the precautions ufed to prevent it; both 
which caufes may produce a considerable error, 
where the denfity of the air to be tried is many times 
lefs than that of common air. For this reafon, I made 
the following experiments. 
I endeavoured to find the weight of the air dis- 
charged from a given quantity of zinc by folution in 
the vitriolic acid, in the manner reprefented in Fig. 4. 
A is a bottle filled near full with oil of vitriol diluted 
with about fix times its weight of water: B is a glafs 
tube fitted into its mouth, and fecured with lute: C is 
a glafs cylinder faftened on the end of the tube, and 
fecured alfo with lute. The cylinder has a fmall hole 
at its upper end to let the inflammable air efcape, and 
is filled with dry pearl-aShes in coarfe powder. The 
whole apparatus, together with the zinc, which was 
intended to be put in, and the lute which was to be 
ufed in fecuring the tube to the neck of the bottle, 
were firfl weighed carefully; its weight was 11930 
grains. The zinc was then putin, and the tube put in 
its place. By this means, the inflammable air was 
made to pafs through the dry pearl-allies ; whereby it 
mull; have been pretty effectually deprived of any acid 
Vol. LVI. X or 
