Lucretius, De Rerum Natura
William Ellery Leonard, Ed.

     Again, from following cause it comes to pass
That yon swift golden hue of liquid fire
Darts downward to the earth: because the clouds
Themselves must hold abundant seeds of fire;
For, when they be without all moisture, then
They be for most part of a flamy hue
And a resplendent. And, indeed, they must
Even from the light of sun unto themselves
Take multitudinous seeds, and so perforce
Redden and pour their bright fires all abroad.
And therefore, when the wind hath driven and thrust,
Hath forced and squeezed into one spot these clouds,
They pour abroad the seeds of fire pressed out,
Which make to flash these colours of the flame.
Likewise, it lightens also when the clouds
Grow rare and thin along the sky; for, when
The wind with gentle touch unravels them
And breaketh asunder as they move, those seeds
Which make the lightnings must by nature fall;
At such an hour the horizon lightens round
Without the hideous terror of dread noise
And skiey uproar.