A. 1st—When hydrogen of fuel and oxygen of air condense into water, a vacuum is made; and the air is disturbed, as a pond would be, if a pail of water were taken out of it: and
2ndly—When the carbon of fuel and oxygen of air expand into carbonic acid gas, the air is again disturbed, as it would be by the explosion of gunpowder.
Q. How does fire condense hydrogen and oxygen into water?
A. The hydrogen of fuel and oxygen of air (liberated by combustion) combining together, condense into water.
Q. How does fire expand carbon into carbonic acid gas?
A. The carbon of fuel and oxygen of air (combining together in combustion) expand into a gas, called carbonic acid.
Q. Why is a fire (after it has been long burning) red hot?
A. When coals are heated throughout, the carbon is so completely mixed with the oxygen of the air, that the whole surface is in a state of combustion, and therefore red hot.
Q. In a blazing fire, why is the upper surface of the coals black, and the lower surface red?
A. Carbon (being very solid) requires a great degree of heat to make it unite with the oxygen of the air. When fresh coals are put on, their under surface is heated before the upper surface; and one is red (or in a state of combustion), while the other is black.