by converging the rays to a focus
will set combustible bodies on fire, without its own temperature being
raised.

EMILY.

Yet, Mrs. B., if I hold a piece of glass near the fire it is almost
immediately warmed by it; the glass therefore must retain some of the
caloric radiated by the fire? Is it that the solar rays alone pass
freely through glass without paying tribute? It seems unaccountable that
the radiation of a common fire should have power to do what the sun's
rays cannot accomplish.

MRS. B.

It is not because the rays from the fire have more power, but rather
because they have less, that they heat glass and other transparent
bodies. It is true, however, that as you approach the source of heat the
rays being nearer each other, the heat is more condensed, and can
produce effects of which the solar rays, from the great distance of
their source, are incapable. Thus we should find it impossible to roast
a joint of meat by the sun's rays, though it is so easily done by
culinary heat. Yet caloric emanated from burning bodies, which is
commonly called _culinary heat_, has neither the intensity nor the
velocity of solar rays. All caloric, we have said, is supposed to
proceed originally from the sun; but after having been incorporated with
terrestrial bodies, and again given out by them, though its nature is
not essentially altered, it retains neither the intensity nor the
velocity with which it first emanated from that luminary; it has
therefore not the power of passing through transparent mediums, such as
glass and water, without being partially retained by those bodies.

EMILY.

I recollect that in the experiment on the reflection of heat, the glass
skreen which you interposed between the burning taper and the mirror,
arrested the rays of caloric, and suffered only those of light to pass
through it.

CAROLINE.

Glass windows, then, though they cannot be heated by the sun shining on
them, may be heated internally by a fire in the room? But, Mrs. B.,
since the atmosphere is not warmed by the solar rays passing through it,
how does it obtain heat; for all the fires that are burning on the
surface of the earth would contribute very little towards warming it?

EMILY.

The radiation of heat is not confined to burning bodies: for all bodies,
you know, have that property; therefore, not only every thing upon the
surface of the earth, but the earth itself, must radiate heat; and this
terrestrial caloric, not havin