e necessarily heated in consequence of that absorption;
to discover which of the various materials that can be employed for
constructing fire-places are best adapted for that purpose, we have only
to find, by an experiment very easy to be made, what bodies acquire
least heat, when exposed to the direct rays of a clear fire; for those
which are least heated evidently absorb the least, and consequently
reflect the most radiant heat. And hence it appears that iron, and in
general metals of all kinds, which are well known to grow very hot when
exposed to the rays projected by burning fuel, are to be reckoned among
the very worst materials that it is possible to employ in the
construction of fire-places. Perhaps the best materials are fire-stone
and common bricks and mortar. These substances are fortunately very
cheap, and it is not easy to say to which of the two the preference
ought to be given. When bricks are used, they should be covered with a
thin coating of plaster, which, when perfectly dry, should be
white-washed. The fire-stone should likewise be white-washed, when that
is used; and every part of the fire-place which does not come into
actual contact with the burning fuel should be kept as white and clean
as possible. The bringing forward of the fire into the room, or rather
bringing it nearer to the front of the opening of the fire-place, and
the diminishing of the throat of the chimney, being two objects
principally had in view in the alterations of fire-places recommended,
it is evident that both these may be attained merely by bringing forward
the back of the chimney. It will then remain to be determined how far
the back should be brought forward. This point will be limited by the
necessity of leaving a proper passage for the smoke. Now, as this
passage, which in its narrowest part is called the throat of the
chimney, ought, for reasons before stated, to be immediately or
perpendicularly over the fire, it is evident that the back of the
chimney should be built perfectly upright. To determine therefore the
place of the new back, nothing more is necessary than to ascertain how
wide the throat of the chimney ought to be left. This width is
determined by Count Rumford from numerous experiments, and comparing all
circumstances, to be four inches. Therefore, supposing the breast of the
chimney, or the wall above the mantle, to be nine inches thick, allowing
four inches for the width of the throat, this will give thirte