Q. Why will the air swell, if the bladder be laid before the fire?
A. Because the heat of the fire gets between the particles of air, and drives them further apart from each other; which causes the bladder to expand.
Q. Why do unslit chestnuts crack with a loud noise, when roasted?
A. Chestnuts contain a great deal of air, which is expanded by the heat of the fire; and, as the thick rind prevents the air from escaping, it violently bursts through, slitting the rind, and making a great noise.
Q. What occasions the loud crack or report which we hear?
A. 1st—The sudden bursting of the rind makes a report, in the same way as a piece of wood or glass would do, if snapped in two: and
2ndly—The escape of hot air from the chestnut makes a report also, in the same way as gunpowder, when it escapes from a gun.
Q. Why does the sudden bursting of the rind, or snapping of a piece of wood, make a report?
A. As the attraction of the parts is suddenly overcome, a violent jerk is given to the air; this jerk produces rapid undulations in the air, which (striking upon the ear) give the brain the sensation of sound.