late of glass must be interposed between the mirrors; for
light, you know, passes with great facility through glass, whilst the
transmission of caloric is almost wholly impeded by it. We shall find,
however, in this experiment, that some few of the calorific rays pass
through the glass together with the light, as the thermometer rises a
little; but, as soon as the glass is removed, and a free passage left to
the caloric, it will rise considerably higher.

EMILY.

This experiment, as well as that of Dr. Herschell's, proves that light
and heat may be separated; for in the latter experiment the separation
was not perfect, any more than in that of Mr. Pictet.

CAROLINE.

I should like to repeat this experiment, with the difference of
substituting a cold body instead of the hot one, to see whether cold
would not be reflected as well as heat.

MRS. B.

That experiment was proposed to Mr. Pictet by an incredulous philosopher
like yourself, and he immediately tried it by substituting a piece of
ice in the place of the heated bullet.

CAROLINE.

Well, Mrs. B., and what was the result?

MRS. B.

That we shall see; I have procured some ice for the purpose.

EMILY.

The thermometer falls considerably!

CAROLINE.

And does not that prove that cold is not merely a _negative_ quality,
implying simply an inferior degree of heat? The cold must be _positive_,
since it is capable of reflection.

MRS. B.

So it at first appeared to Mr. Pictet; but upon a little consideration
he found that it afforded only an additional proof of the reflection of
heat: this I shall endeavour to explain to you.

According to Mr. Prevost's theory, we suppose that all bodies whatever
radiate caloric; the thermometer used in these experiments therefore
emits calorific rays in the same manner as any other substance. When its
temperature is in equilibrium with that of the surrounding bodies, it
receives as much caloric as it parts with, and no change of temperature
is produced. But when we introduce a body of a lower temperature, such
as a piece of ice, which parts with less caloric than it receives, the
consequence is, that its temperature is raised, whilst that of the
surrounding bodies is proportionally lowered.

EMILY.

If, for instance, I was to bring a large piece of ice into this room,
the ice would in time be melted, by absorbing caloric from the general
radiation which is going on throughout the room; and as it would
contribut