e now separate as vapour which can
recondense. After condensation gravity comes into effectual play,
pulling the showers down upon the hills, and the rivers thus created
through their gorges to the sea. Every raindrop which smites the
mountain produces its definite amount of heat; every river in its
course develops heat by the clash of its cataracts and the friction of
its bed. In the act of condensation, moreover, the molecular work of
vaporisation is accurately reversed. 'Compare, then, the primitive
loss of solar warmth with the heat generated by the condensation of
the vapour, and by the subsequent fall of the water from cloud to sea.
They are mathematically equal to each other. No particle of vapour
was formed and lifted without being paid for in the currency of solar
heat; no particle returns as water to the sea without the exact
quantitative restitution of that heat. There is nothing gratuitous in
physical nature, no expenditure without equivalent gain, no gain
without equivalent expenditure. With inexorable constancy the one
accompanies the other, leaving no nook or crevice between them for
spontaneity to mingle with the pure and necessary play of natural
force. Has this uniformity of nature ever been broken? The reply is:

'Not to the knowledge of science.'

What has been here stated regarding heat and gravity applies to the
whole of inorganic nature. Let us take an illustration from
chemistry. The metal zinc may be burnt in oxygen, a perfectly
definite amount of heat being produced by the combustion of a given
weight of the metal. But zinc may also be burnt in a liquid which
contains a supply of oxygen--in water, for example. It does not in
this case produce flame or fire, but it does produce heat which is
capable of accurate measurement. But the heat of zinc burnt in water
falls short of that produced in pure oxygen, the reason being that to
obtain its oxygen from the water the zinc must first dislodge the
hydrogen. It is in the performance of this molecular work that the
missing heat is absorbed. Mix the liberated hydrogen with oxygen and
cause them to recombine; the heat developed is mathematically equal to
the missing heat. Thus in pulling the oxygen and hydrogen asunder an
amount of heat is consumed which is accurately restored by their
reunion.

This leads up to a few remarks upon the Voltaic battery. It is not my
design to dwell upon the technical features of this wonderful
instrum