Heat is a motion; expansive, restrained, and acting in its strife upon the smaller particles of bodies. But the expansion is thus modified; while it expands all ways, it has at the same time an inclination upward. And the struggle in the particles is modified also; it is not sluggish, but hurried and with violence.
The New Organon (1620) in James Spedding, Robert Ellis and Douglas Heath (eds.), The Works of Francis Bacon (1887–1901), Vol. 4, 154–5