nd ingenious
apparatus, by means of which he made these experiments. This cubical tin
vessel or canister, has each of its sides externally covered with
different materials; the one is simply blackened; the next is covered
with white paper; the third with a pane of glass, and in the fourth the
polished tin surface remains uncovered. We shall fill this vessel with
hot water, so that there can be no doubt but that all its sides will be
of the same temperature. Now let us place it in the focus of one of the
mirrors, making each of its sides front it in succession. We shall begin
with the black surface.

CAROLINE.

It makes the thermometer which is in the focus of the other mirror rise
considerably. Let us turn the paper surface towards the mirror. The
thermometer falls a little, therefore of course this side cannot emit or
radiate so much caloric as the blackened side.

EMILY.

This is very surprising; for the sides are exactly of the same size, and
must be of the same temperature. But let us try the glass surface.

MRS. B.

The thermometer continues falling, and with the plain surface it falls
still lower; these two surfaces therefore radiate less and less.

CAROLINE.

I think I have found out the reason of this.

MRS. B.

I should be very happy to hear it, for it has not yet (to my knowledge)
been accounted for.

CAROLINE.

The water within the vessel gradually cools, and the thermometer in
consequence gradually falls.

MRS. B.

It is true that the water cools, but certainly in much less proportion
than the thermometer descends, as you will perceive if you now change
the tin surface for the black one.

CAROLINE.

I was mistaken certainly, for the thermometer rises again now that the
black surface fronts the mirror.

MRS. B.

And yet the water in the vessel is still cooling, Caroline.

EMILY.

I am surprised that the tin surface should radiate the least caloric,
for a metallic vessel filled with hot water, a silver teapot, for
instance, feels much hotter to the hand than one of black earthen ware.

MRS. B.

That is owing to the different power which various bodies possess for
_conducting_ caloric, a property which we shall presently examine. Thus,
although a metallic vessel feels warmer to the hand, a vessel of this
kind is known to preserve the heat of the liquid within, better than one
of any other materials; it is for this reason that silver teapots make
better tea than those of earthen