Heat.
Why does hay, if stacked when damp, take fire?
Because the moisture elevates the temperature sufficiently to produce putrefaction, and the ensuing chemical action causes sufficient heat to continue the process; the quantity of matter being also great, the heat is proportional.
Why is the air warm in misty or rainy weather?
Because of the liberation of the latent heat from the precipitated vapour.
Why is heated air thinner or lighter than cold air?
Because it is a property of heat to expand all bodies; or rather we should say, that we call air hot or cold, according as it naturally is more or less expanded.
Why is a tremulous motion observable over chimney-pots, and slated roofs which have been heated by the sun?
Because the warm air rises, and its refracting power being less than that of the colder air, the currents are rendered visible by the distortion of objects viewed through them.
Within doors, a similar example occurs above the foot-lights of the stage of a theatre; the flame of a candle, or the smoke of a lamp.