further heated. On cooling again, it contracts,
and returns successively through all the
degrees of its dilatation till it arrives at its first
bulk, being never of the same magnitude two
minutes successively. Gold, when fused, takes
up more space than before; mercury, placed in
a narow tube over the fire, will ascend to thirty
times its former height.

Boerhaave discovered as the Law of this Expansion
—First, that the same degree of Fire rarefies
fluids sooner, and in greater degree, than it does
solids. Without this law, the thermometer
would be useless, since the cavity of the tube
would then be dilated in the same proportion as
the fluid is rarefied. Secondly, that the lighter
the fluid, the more it is dilated by Fire. Air,
the lightest of all fluids known to him, expands
the most; after air, spirit of wine. He held
that all the motion in nature arises from Fire
alone; taking this away, all things become
immovable. At the absence of only a certain
degree of Fire, all oils, waters, spirits,
vegetables, and animals, become hard, rigid, and
inert. If the greatest degree of cold were
arrived at, and all Fire were absolutely taken
away, all Nature would grow into one concrete
body, solid as gold and hard as diamond: on the
reapplication of Fire it would recover its former
mobility.

Boerhaave's elemental Fire needs no air nor
pabulum to sustain or preserve it. If a quantity
of any essential aromatic oil be poured in
vacuo upon spirit of nitre, there will immediately
arise a huge Fire, to the great danger of
the bystanders. He adds that the effects of
elementary Fire may be increased in divers ways.
By a swift agitation of one body against another:
every one knows that, in solids, a vehement
attrition of a flint and steel will produce sparks;
in fluids, cream, by long churning to separate
the butter, will grow sensibly warm, as will be
made still more apparent by the use of a
thermometer. A knife, whetted briskly on a dry
rough stone, yields sparks of Fire. In these
and other manners, he continues, it does not
appear that any Fire is generated out of what
was not Fire before.

If, in a severe winter's day, we rub a plate of
gold briskly against another gold plate, they
will both gradually grow hotter and hotter, until
at length they become red-hot, and at the point of
melting; and yet, all this time the plates lose
nothing of their weight, but swell and grow bigger
in all their dimensions. Hence it follows that
the particles of the gold are not converted by
the friction into Fire. The Fire existed before;
and all the effect of the friction is to collect and
bring together a quantity thereof, before
dispersed throughout the atmosphere. There is no
making or producing of Fire de novo. All we
can do, is, if insensible, to render it sensible: to
collect it out of a greater space into a lesser, and
to direct and determine it to certain places.

Besides the solar, Boerhaave likewise
admitted a subterranean Fire, which manifests
itself on digging underground. Arriving at a
certain depth, viz. forty or fifty feet, things begin
to grow warmer, so that no ice can there subsist.
At a greater depth, air is so hot as to take away
respiration. Whence he infers that there is
another source of Fire, or another sun, in the
bosom of the earth, which gives motion and life
to every thing growing in or upon the globe
and even that the centre of the earth is mere
Fire; which Fire is argued to be perpetual, from
volcanoes which have been known to cast up
Fire from the earliest accounts of time.
Boerhaave's speculations are a great stride towards
divesting Fire of its material and substantial
character, although one of his commentators
calls them "a pompous galimatias." His
elementary Fire is hardly a thing.

The chemists of old used four principal degrees
of Fire in their operations. The first was equal
to the natural heat of the human body, or rather
of a hen brooding on her eggs, which was the
standard employed. Accordingly, this first
degree was measured by applying a thermometer
to a hen. Some chemists, by keeping a Fire
continually to this degree, have hatched chickens.
The second degree of Fire was that which gives
a man pain, like the heat of a scorching summer's
sun which chafes and inflames the skin, and even
sometimes raises blisters, but does not destroy
or consume the parts. It makes the serum of
blood and the white of eggs to coagulate, and
so occasions deadly inflammations.

The third degree of Fire was that of boiling
water, which separates and destroys the parts of
bodies. This degree was thought perfectly
stable; for water, when once it boils, is at its
utmost degree of heat (at that spot, and with
an evaporating surface), and cannot be raised
a jot further by any augmentation of fire
or fuel. They did not seem to know that
boiling water at the top of Mont Blanc is less
hot than boiling water at the level of the sea;
nor was Papin's Digester yet a generally
accomplished fact. The fourth degree was that which
melts metals and destroys everything else. It
was too vehement to be estimated by the
thermometer, which, itself would be demolished by
so ardent a heat. Wedgewood's pyrometer was
neither born nor thought of, so that the fourth
degree was only determinable by its effects in
the fusion of metals. As the heat of boiling
water could not be increased, so neither could
that of melted metals. [But how did they know
that, if they had no means of measuring the heat
of melted metals?] This was the last degree
known to the ancient chemists. Later philosophers
reckoned a fifth degree of Fire; that
whereby gold is made to emit fumes, and evaporate,
  In 1690, M. Tschirnhausen's burning-glass
rendered everything, even gold itself,
volatile.

Caloric is the actor of all-work who next
makes his entrance on our fiery scene. In fact,
caloric is one of the leading stars of the close of
the last century and the commencement of this.
The name of caloric is given to the agent
which causes in us the sensation of heat,
acting also on inorganic bodies. It is caloric
which melts ice, boils water, and makes iron