What Democritus Saw and You Can Too

Democritean pedesis (from Greek: πήδησις "leaping") is the assumably random movement of particles suspended in a fluid (i.e. a liquid such as water or a gas such as air) or the mathematical model used to describe such random movements, often called a particle theory.

"... his [Democritus's] ... atoms are infinite in number ... and [he] compares them to the motes of air which we see in shafts of light coming through windows ...." -- Aristotle, philosopher, On the Soul, Book I, 350 B.C.

Lucretius described Democritean pedesis in his scientific poem On the Nature of Things written in 50 B.C.

"And of this fact (as I record it here)
An image, a type goes on before our eyes
Present each moment; for behold whenever
The sun's light and the rays, let in, pour down
Across dark halls of houses: thou wilt see
The many mites in many a manner mixed
Amid a void in the very light of the rays,
And battling on, as in eternal strife,
And in battalions contending without halt,
In meetings, partings, harried up and down.
From this thou mayest conjecture of what sort
The ceaseless tossing of primordial seeds
Amid the mightier void- at least so far
As small affair can for a vaster serve,
And by example put thee on the spoor
Of knowledge. For this reason too 'tis fit
Thou turn thy mind the more unto these bodies
Which here are witnessed tumbling in the light:
Namely, because such tumblings are a sign
That motions also of the primal stuff
Secret and viewless lurk beneath, behind.
For thou wilt mark here many a speck, impelled
By viewless blows, to change its little course,
And beaten backwards to return again,
Hither and thither in all directions round."
-- T. Lucretius Carus, philosopher poet, On The Nature of Things, Book II, 50 B.C.

In Democritus's lost botanical work titled Causes Affecting Seeds, and Plants, and Fruits, Democritus described pedesis observed under the microscope.

"It would surely be absurd to imagine that the men who could make these observations had not the curiosity or the ability to make many others of which the memory is lost. Indeed, the idea that the Greeks were not observers is ludicrously wrong, as is proved by the anatomical accuracy of their sculpture, which bears witness to trained habits of observation, while the Hippokratean corpus contains models of scientific observation at its best. We know, then, that the Greeks could observe well, and we know that they were curious about the world. Is it conceivable that they did not use their powers of observation to gratify that curiosity? It is true that they had not our instruments of precision; but a great deal can be discovered by the help of very simple apparatus." -- John Burnet, classicist, Early Greek Philosophy, 1892

"Democritus probably deduced the large size of the saline atom from observing that when saline water is filtered the salt is left behind on the surface." -- J.B. McDiarmid, philologist, Theophrastus De Sensibus 66: Democritus' Explanation of Salinity, The American Journal of Philology, Volume 80, Number 1, Pages 56-66, 1959

Ford, B.J., Brownian Movement in Clarkia Pollen: A Reprise of the First [sic] Observations, The Microscope, Volume 40, Number 4, Pages 235-241, 1992

Pearle, P., et al., What Brown Saw and You Can Too, American Journal of Physics, Volume 78, Issue 12, Page 1278, Dec 2010