ken place, the black canister, both receiving and
giving out 400 rays, and the metallic one 200, no change of temperature
will take place.

EMILY.

I now understand it extremely well. But what becomes of the surplus of
calorific rays, which good radiators emit and bad radiators refuse to
receive; they must wander about in search of a resting-place?

MRS. B.

They really do so; for they are rejected and sent back, or, in other
words, _reflected_ by the bodies which are bad radiators of caloric; and
they are thus transmitted to other bodies which happen to lie in their
way, by which they are either absorbed or again reflected, according as
the property of reflection, or that of absorption, predominates in these
bodies.

CAROLINE.

I do not well understand the difference between radiating and reflecting
caloric, for the caloric that is reflected from a body proceeds from it
in straight lines, and may surely be said to radiate from it?

MRS. B.

It is true that there at first appears to be a great analogy between
_radiation_ and _reflection_, as they equally convey the idea of the
transmission of caloric.

But if you consider a little, you will perceive that when a body
_radiates_ caloric, the heat which it emits not only proceeds from, but
has its origin in the body itself. Whilst when a body _reflects_
caloric, it parts with none of its own caloric, but only reflects that
which it receives from other bodies.

EMILY.

Of this difference we have very striking examples before us, in the tin
vessel of water, and the concave mirrors; the first radiates its own
heat, the latter reflect the heat which they receive from other bodies.

CAROLINE.

Now, that I understand the difference, it no longer surprises me that
bodies which radiate, or part with their own caloric freely, should not
have the power of transmitting with equal facility that which they
receive from other bodies.

EMILY.

Yet no body can be said to possess caloric of its own, if all caloric is
originally derived from the sun.

MRS. B.

When I speak of a body radiating its own caloric, I mean that which it
has absorbed and incorporated either immediately from the sun's rays, or
through the medium of any other substance.

CAROLINE.

It seems natural enough that the power of absorption should be in
opposition to that of reflection, for the more caloric a body receives,
the less it will reject.

EMILY.

And equally so that the power of ra