Lucretius "On the Nature of Things" Book 2, lines 113-140:

It occurs to be that this activity is mirrored and reflected in a phenomenon of our everyday experience.  Watch carefully whenever shafts of streaming sunlight are allowed to penetrate a darkened room.  You will observe many minute particles mingling in many ways in every part of the space illuminated by the rays and, as though engaged in ceaseless combat, warring and fighting by squadrons with never a pause, agitated by frequent unions and disunions.  You can obtain from this spectacle a conception of the perpetual restless movement of the primary elements in the vast void, insofar as a trivial thing can exemplify important matters and put us on the track of knowledge.

A further reason why you should devote particular attention to these particles that are seen to be in commotion in the sun's rays is that such a commotion also implies the existence of movements of matter that are secret and imperceptible.  For you will observe many of those particles, under the force of unseen blows, changing course and being forcible turned back, now this way, now that way, in every direction.  It is evident that they all derive this random movement from the atoms.  First, the primary elements of things move of their own accord; next, the smallest atomic aggregates, which are, one might say, nearest in force to the atoms, are impelled by the impact of the unseen blows of the atoms; and they themselves in their turn assail slightly larger compounds.  So the scale of movement ascends from the atoms and by degrees passes within the range of our senses, so that eventually movement is extended to those particles that we can perceive in the sun's light, although the blows that cause their movement are imperceptible.