degree, the attraction of cohesion; and the body, from a solid, is then
converted into a fluid.

MRS. B.

This is the case whenever a body is fused or melted; but if you add
caloric to a liquid, can you tell me what is the consequence?

CAROLINE.

The caloric forces itself in greater abundance between the particles of
the fluid, and drives them to such a distance from each other, that
their attraction of aggregation is wholly destroyed: the liquid is then
transformed into vapour.

MRS. B.

Very well; and this is precisely the case with boiling water, when it is
converted into steam or vapour, and with all bodies that assume an
aeriform state.

EMILY.

I do not well understand the word aeriform?

MRS. B.

Any elastic fluid whatever, whether it be merely vapour or permanent
air, is called aeriform.

But each of these various states, solid, liquid, and aeriform, admit of
many different degrees of density, or consistence, still arising
(chiefly at least) from the different quantities of caloric the bodies
contain. Solids are of various degrees of density, from that of gold, to
that of a thin jelly. Liquids, from the consistence of melted glue, or
melted metals, to that of ether, which is the lightest of all liquids.
The different elastic fluids (with which you are not yet acquainted) are
susceptible of no less variety in their degrees of density.

EMILY.

But does not every individual body also admit of different degrees of
consistence, without changing its state?

MRS. B.

Undoubtedly; and this I can immediately show you by a very simple
experiment. This piece of iron now exactly fits the frame, or ring, made
to receive it; but if heated red hot, it will no longer do so, for its
dimensions will be so much increased by the caloric that has penetrated
into it, that it will be much too large for the frame.

The iron is now red hot; by applying it to the frame, we shall see how
much it is dilated.

EMILY.

Considerably so indeed! I knew that heat had this effect on bodies, but
I did not imagine that it could be made so conspicuous.

MRS. B.

By means of this instrument (called a Pyrometer) we may estimate, in the
most exact manner, the various dilatations of any solid body by heat.
The body we are now going to submit to trial is this small iron bar;
I fix it to this apparatus, (PLATE I. Fig. 1.) and then heat it by
lighting the three lamps beneath it: when the bar expands, it increases
in length