"[A]s it is an invariable rule among all fluids, that those which are lightest rise upwards, and at length swim upon the top of such as are more weighty... so... that when any particles of the same fluid are... rendered lighter, or more weighty, than other parts... they either rise to the top, and give place to the more dense and weighty parts of it, or sink to the bottom, and force the warmer fluid to the surface."