point. When I took the flask from the lamp, I observed to you
that the upper part of it was filled with vapour; this being compelled
to yield its caloric to the cold water, was again condensed into water--
What, then, filled the upper part of the flask?

EMILY.

Nothing; for it was too well corked for the air to gain admittance, and
therefore the upper part of the flask must be a vacuum.

MRS. B.

The water below, therefore, no longer sustains the pressure of the
atmosphere, and will consequently boil at a much lower temperature.
Thus, you see, though it had lost many degrees of heat, it began boiling
again the instant the vacuum was formed above it. The boiling has now
ceased, the temperature of the water being still farther reduced; if it
had been ether, instead of water, it would have continued boiling much
longer, for ether boils, under the usual atmospheric pressure, at a
temperature as low as 100 degrees; and in a vacuum it boils at almost
any temperature; but water being a more dense fluid, requires a more
considerable quantity of caloric to make it evaporate quickly, even when
the pressure of the atmosphere is removed.

EMILY.

What proportion of vapour can the atmosphere contain in a state of
solution?

MRS. B.

I do not know whether it has been exactly ascertained by experiment; but
at any rate this proportion must vary, both according to the temperature
and the weight of the atmosphere; for the lower the temperature, and the
greater the pressure, the smaller must be the proportion of vapour that
the atmosphere can contain.

To conclude the subject of free caloric, I should mention _Ignition_, by
which is meant that emission of light which is produced in bodies at a
very high temperature, and which is the effect of accumulated caloric.

EMILY.

You mean, I suppose, that light which is produced by a burning body?

MRS. B.

No: ignition is quite independent of combustion. Clay, chalk, and indeed
all incombustible substances, may be made red hot. When a body burns,
the light emitted is the effect of a chemical change which takes place,
whilst ignition is the effect of caloric alone, and no other change than
that of temperature is produced in the ignited body.

All solid bodies, and most liquids, are susceptible of ignition, or, in
other words, of being heated so as to become luminous; and it is
remarkable that this takes place pretty nearly at the same temperature
in all bodies, that is, at ab