A. The iron stove contracts again, as soon as the fire is removed; and (as it shrinks into a smaller space) the parts rub against each other again, and the bricks are again disturbed; and this produces a cracking noise.
Q. Why does the plaster round a stove crack and fall away?
A. When the fire is lighted, the iron-work (which expands more than the brick-work and plaster) pushes away the bricks and plaster: but when the fire is put out, the metal shrinks again, and leaves the “setting” behind.
Q. Why does the plaster fall away?
A. As a chink is left (between the “setting” and the stove), the plaster will frequently fall away from its own weight.
Q. What other cause contributes to bring the plaster down?
A. As the heat of the fire varies, the size of the iron stove varies also; and this swelling and perpetually contracting, keeps up such a constant disturbance about the plaster, that it cracks and falls off, leaving the fire-place very unsightly.
Q. Why does the mercury of a thermometer rise in hot weather?
A. Heat expands the metal; and as the metal is increased in bulk, it occupies a larger space, (or, in other words, rises higher in the tube.)