philosophers, even by those who may have but a superficial
knowledge. because it can be proven by exhausting, if not all,
at any rate the greater part of, the air contained in a glass
vessel, which, if weighed before and after the air has been
exhausted, will be found materially reduced in weight. Then I
found out how much the air weighed in itself in the following
manner. I procured a large vessel of glass, whose neck could be
closed or opened by means of a tap, and holding it open I warmed
it over a fire, so that the air inside it becoming rarified, the
major part was forced out; then quickly shutting the tap to
prevent the re-entry I weighed it; which done, I plunged its
neck in water, resting the whole of the vessel on the surface of
the water, then on opening the tap the water rose in the vessel
and filled the greater part of it. I lifted the neck out of the
water, released the water contained in the vessel, and measured
and weighed its quantity and density, by which I inferred that a
certain quantity of air had come out of the vessel equal in bulk
to the quantity of water which had entered to refill the portion
abandoned by the air. I again weighed the vessel, after I had
first of all well dried it free of all moisture, and found it
weighed one ounce more whilst it was full of air than when it
was exhausted of the greater part, so that what it weighed more
was a quantity of air equal in volume to the water which took
its place. The water weighed 640 ounces, so I concluded that
the weight of air compared with that of water was 1 to 640--that
is to say, as the water which filled the vessel weighed 640
ounces, so the air which filled the same vessel weighed one
ounce.'

Having thus detailed the method of exhausting air from a vessel,