[ 128 I 
of water, compofed of the fame cx3nftltuent parti- 
cles ; and that the veficle, by this means, becomes 
fpecifically lighter than air. But I think, that this 
cannot be done fo eafily, as it has been generally ima- 
gined ; and when done, it will not be fufficient for 
the purpofe ; which will appear from the following 
conliderations. 
Firft, the great difficulty in forming thofe veficles, 
efpecially of the particles of dry bodies carried off 
by exhalation, and filling them with rarified air, 
while the exterior air remains condenfed. Secondly, 
That there is not any allowance made for the weight 
of the included air. Thirdly, The conftituent par- 
ticles of water are but very little (if at all) altered in 
their fpecific gravity. Fourthly, That this thin veficle 
can never be a fufficient boundary between the ex- 
terior condenfed air and the interior air, fo exceed- 
ingly rarified. And therefore as the denfity of air 
is proportional to the force compreffing it, it follows, 
that the interior air muff be fuddenly compreffed 
(and the veficle with it), until it be of equal denfity 
with the exterior air ; in which ftate the veficle is 
not more fitted for afcending, than if the fame con- 
ftituent watry particles were formeddnto a fpherical 
drop. 
For here I muff beg leave to fet afide a falfe rea- 
foning, which I have found in many approved writers 
on this fubjed, who affert, that the fpecific gravity 
of bodies is altered by a mere increafe of furface, 
upon tiheir being divided into minute parts. Which 
is wrong ^ for the fpecific gravity of any body is only 
to be altered by making that body occupy a greater 
or Jefs proportion of fpace. It is evident, that upon 
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