On the nature of things (books 1 - 3)   

Which, waked by nimble fingers, on the strings
The sweet musicians fashion; or suppose
That same-shaped atoms through men's nostrils pierce
When foul cadavers burn, as when the stage
Is with Cilician saffron sprinkled fresh,
And the altar near exhales Panchaean scent;
Or hold as of like seed the goodly hues
Of things which feast our eyes, as those which sting
Against the smarting pupil and draw tears,
Or show, with gruesome aspect, grim and vile.
For never a shape which charms our sense was made
Without some elemental smoothness; whilst
Whate'er is harsh and irksome has been framed
Still with some roughness in its elements.
Some, too, there are which justly are supposed
To be nor smooth nor altogether hooked,
With bended barbs, but slightly angled-out,
To tickle rather than to wound the sense-
And of which sort is the salt tartar of wine
And flavours of the gummed elecampane.
Again, that glowing fire and icy rime
Are fanged with teeth unlike whereby to sting
Our body's sense, the touch of each gives proof.
For touch- by sacred majesties of gods!-
Touch is indeed the body's only sense-
Be't that something in-from-outward works,
Be't that something in the body born
Wounds, or delighteth as it passes out
Along the procreant paths of Aphrodite;
Or be't the seeds by some collision whirl
Disordered in the body and confound
By tumult and confusion all the sense-
As thou mayst find, if haply with the hand
Thyself thou strike thy body's any part.
On which account, the elemental forms
Must differ widely, as enabled thus
To cause diverse sensations.
And, again,
What seems to us the hardened and condensed
Must be of atoms among themselves more hooked,
Be held compacted deep within, as 'twere
By branch-like atoms- of which sort the chief
Are diamond stones, despisers of all blows,
And stalwart flint and strength of solid iron,
And brazen bars, which, budging hard in locks,
Do grate and scream. But what are liquid, formed
Of fluid body, they indeed must be
Of elements more smooth and round- because
Their globules severally will not cohere:
To suck the poppy-seeds from palm of hand
Is quite as easy as drinking water down,
And they, once struck, roll like unto the same.
But that thou seest among the things that flow
Some bitter, as the brine of ocean is,
Is not the least a marvel...
For since 'tis fluid, smooth its atoms are
And round, with painful rough ones mixed therein;
Yet need not these be held together hooked:
In fact, though rough, they're globular besides,
Able at once to roll, and rasp the sense.
And that the more thou mayst believe me here,
That with smooth elements are mixed the rough
(Whence Neptune's salt astringent body comes),
There is a means to separate the twain,