one against the other; that motion is
converted into heat, when it is suddenly stopped;
and that heat is converted into motion when
—under the piston of a steam-engine—the
vapour of water, expanding in a vacuum, is
cooled by the expansion, in such a way that
the lost caloric is seen to be transformed into
motion.

It is only of late that this principle has
been propounded in its complete generality.
About the year eighteen hundred,
Montgolfier, the celebrated aëronaut, caught a
glimpse of it, by recognising the possibility
of the reciprocal conversion of motion and
caloric. But this induction of genius was so
completely beyond the limits and in advance
of received ideas, that it did not meet with
the slightest response at the time. Even in
eighteen hundred and fifty-five, science was
so little familiarised to the proposition, that
the most celebrated natural philosophers
could scarcely comprehend the drift of
Monsieur Léon Foucault's brilliant experiment,
by which he proved that a mass of copper in
rotatory motion, suddenly arrested at a
distance and without contact by means of a
magnet, becomes heated, and that to a
degree proportional to the retardation of its
motion.

What has been said of the reciprocal
conversion of heat and motion into each other,
is applicable to the rest of the physical forces.
As motion generates heat, so can it also
generate magnetism, light, and chemical
affinity. It is generally known that if, instead
of rubbing against each other two homogenous
bodies, such as two pieces of wood or two
bits of iron, the friction is made to take place
between two heterogeneous bodies, such as a
stick of sealing-wax and a scrap of woollen
cloth, electricity is produced. Monsieur
Babinet has modified this experiment into
an original and striking form, which is
worth describing, in order that any one
who likes may enjoy the pleasure of repeating it.

On the smooth stopper of a decanter,
balance, in a horizontal position, a rather
heavy walking-stick of wood or cane, in such
a way that it shall easily turn on its resting-
point. Then take a piece of sealing-wax, rub
it briskly on the sleeve of your coat, and
immediately put it very close to one of the ends
of the stick, without actually touching it.
You will find, not without surprise, that the
stick, yielding to the influence of the electricity
detached by the friction, will turn on its
resting-point, and will follow the sealing-wax
as it is gradually withdrawn, so as even to make
a complete revolution, if the operator thinks
fit to carry the experiment so far. In this
case, motion has generated electricity; and
electricity, in turn, has given birth to motion.
Surely, this is worth all the table-turnings
and hat-spinnings that have ever been
performed in the Old World and in the
New.

It is not always possible, in the present
state of science, to prove that any two forces
whatever, taken at hazard, will produce each
other; it is sometimes necessary to have
recourse to an intermediate force. Thus,
for instance, we cannot yet immediately transform
heat into electricity; whilst, by taking
motion as the intermediate force, that is to
say, by causing a steam-engine to turn a
gigantic plate of glass between a pair of
cushions, we can obtain torrents of electricity.
Mr. Grove is persuaded that the direct and
immediate generation of all the physical
forces by any one of them whatsoever, is
possible. Science appears to be rapidly
advancing towards the establishment of this
capital fact, which would open a completely
new era of discovery. For instance, one of
the crying wants of the day, is cheap electricity ;
we know how to do many things by
electricity which it is most desirable that
we should be able to do. We can make an
electric motive power, to serve the purposes
of a small steam-engine; we can make
hydrogen gas, for lighting, by decomposing
that very cheap article, water. But the
electric horse-power is so much more
expensive to feed than the steam horse-power,
and the water-gas (now being manufactured
at the Invalides, Paris) costs so much more
to make than coal-gas,—all in consequence of
the dearness of electricity,—that they are
only employed by those who can afford to
pay for curiosities and experimental wonders.
But perhaps the day is not far distant when,
by the transformation into electricity of the
heat generated in the furnaces of our
manufactories, we may create a source of
electricity, a veritable voltaic pile fulfilling all
the conditions of power which it is requisite
to give to this marvellous instrument.

Mr. Grove contrived an experiment, too
complicated to be fully described here, in
which the apparatus comprises, in a darkened
box filled with water, a daguerreotype plate,
a galvanometer, and one of Bréquet's metallic
thermometers. By drawing the lid or screen
of the box, light is allowed to fall on the
daguerreotype plate. Immediately, the two
needles are seen to move; one indicating the
presence of an electric current, the other, a
disengagement of heat. One single cause,
light, has therefore produced, on the plate,
a chemical action; in the silver wires, an
electric current; in the bobbin of the
galvanometer, a magnetic current; in the
thermometer, heat; in the needles, motion.
Consequently, in this beautiful experiment, one
force only, light, has generated all the
others, mediately or immediately; namely,
chemical action, electricity, magnetism, heat,
motion.

But the most important characteristic of
this common genesis of the natural forces, is
that it takes place in fixed and definite
proportions, according to a law exactly
similar to that of chemical equivalents. For