place a bottle from which the air is exhausted in a balance and exactly
poise it with a counter-weight, and then open it and let in the air, it
will show at once that the air has weight or gravitates by immediately
descending. Thirdly, if we extend a piece of india-rubber over the end
of a vessel and begin to withdraw the air from it, we shall see the
india-rubber sink in, under the pressure of the air outside, to fill up
the space left vacant by the removal of the included air. The fact that
air gravitates we have already taken for granted in explaining the
ascent of a balloon; and the proofs now given are enough to show that
the cause assumed is a real one. The lighter gas rises and the heavier
sinks by law of gravitation.

_Gravitation and Cohesion._--Unlike the attraction of aggregation, or
cohesion, which acts only between particles separated from each other
by spaces that are imperceptible, gravitation takes effect at distances
which transcend conception, but it diminishes in force as the distance
increases. The law according to which it does so is expressed thus; its
intensity decreases with the square of the distance; that is to say, at
twice the original distance it is 1-4th; at thrice, 1-9th; at four
times, 1-16th, for 4, 9, 16 are the squares respectively of 2, 3, and 4.
To take an instance, a ball which weighs 144 lb. at the surface of the
earth will weigh 1-4th of that, or 36 lb., when it is twice as far from
the centre as it is at the surface; and 1-9th, or 16 lb. when it is
thrice as far; and 1-16th, or 9 lb. when it is four times as far. The
attraction of cohesion, on the other hand, as we say, acts only when the
particles seem almost in contact, and it ceases altogether when once, by
mechanical or other means, the bond is broken, in consequence of the
particles being forced too near, or sundered too far from, one another.

One distinguishing difference between the attraction of gravitation and