its energy; for by means of it you can weld the hardest as well as the
softest substances into one, and two pieces of iron together, no less
than two pieces of wax. It is possible, indeed, by heat to unite two
sufficient waxed corks to one another, so as to be able by means of the
one to draw the other out of a bottle: such, in this case, is the force
of cohesion induced by heat.

The power of cohesion exists between the particles of liquids as well as
those of solids, the only difference being that in solids the particles
are relatively fixed, while in liquids they move freely about one
another, unless indeed when they are attracted to the surface of a
solid--a fact we are familiar with when we dip our finger into a vessel
of water. The cohesive power of liquids is overcome by heat as well as
that of solids, only to a much greater degree, for under it they assume
a new form, acquire new properties, and expand immensely in volume. They
pass into the form of vapour, occupy a thousand times larger area, and
possess an elasticity of compressibility and expansibility they were
destitute of before.

There is a beautiful phenomenon which accompanies the expansion of ether
under the influence of heat. Placed in a flask to which heat is applied,
the ether will go off in vapour; and as the heat increases, the vapour
will gradually light up into a lovely flame. The expansibility of air,
which is vapour in a permanent form, can be shown by experiment. If we
tie up an empty or collapsed bladder, and place it in a vessel over an
air-pump, we may see, as we withdraw the air from the vessel, and so
diminish its pressure, the bladder gradually expand and swell as it does
under inflation.

The cohesive power of water is beautifully illustrated. Have a small