es are
by heat lessened, and would by extreme heat be entirely obliterated.

CAROLINE.

And how do you ascertain the degrees of contraction of Wedgwood's
pyrometer?

MRS. B.

The dimensions of a piece of clay are measured by a scale graduated on
the side of a tapered groove, formed in a brass ruler; the more the clay
is contracted by the heat, the further it will descend into the narrow
part of the tube.

Before we quit the subject of expansion, I must observe to you that, as
liquids expand more readily than solids, so elastic fluids, whether air
or vapour, are the most expansible of all bodies.

It may appear extraordinary that all elastic fluids whatever, undergo
the same degree of expansion from equal augmentations of temperature.

EMILY.

I suppose, then, that all elastic fluids are of the same density?

MRS. B.

Very far from it; they vary in density, more than either liquids or
solids. The uniformity of their expansibility, which at first may appear
singular, is, however, readily accounted for. For if the different
susceptibilities of expansion of bodies arise from their various degrees
of attraction of cohesion, no such difference can be expected in elastic
fluids, since in these the attraction of cohesion does not exist, their
particles being on the contrary possessed of an elastic or repulsive
power; they will therefore all be equally expanded by equal degrees of
caloric.

EMILY.

True; as there is no power opposed to the expansive force of caloric in
elastic bodies, its effect must be the same in all of them.

MRS. B.

Let us now proceed to examine the other properties of free caloric.

Free caloric always tends to diffuse itself equally, that is to say,
when two bodies are of different temperatures, the warmer gradually
parts with its heat to the colder, till they are both brought to the
same temperature. Thus, when a thermometer is applied to a hot body, it
receives caloric; when to a cold one, it communicates part of its own
caloric, and this communication continues until the thermometer and the
body arrive at the same temperature.

EMILY.

Cold, then, is nothing but a negative quality, simply implying the
absence of heat.

MRS. B.

Not the total absence, but a diminution of heat; for we know of no body
in which some caloric may not be discovered.

CAROLINE.

But when I lay my hand on this marble table I feel it _positively_ cold,
and cannot conceive that there is any caloric