Whenever we warm our hands by the fire, we allow the energy radiated by the fire to quicken up the movements of the atoms of which the hands are composed. When we cool any substance, we check those movements.

The difference between a gas and a liquid is that in the former, the atoms and molecules move to and fro in an independent existence, whereas in the latter, they are always in touch with one another, though they are changing partners continually.

Sound is a movement which is handed on from atom to atom in a gas through which the sound is passing, just as a chain of workers pass buckets of water to a fire. The quicker the workers move their hands and arms, the quicker the water moves.

William Henry Bragg Quotes