William Cowper - Epigram

To purify their wine some people bleed
A lamb into the barrel, and succeed;
No nostrum, planters say, is half so good
To make fine sugar, as a negro's blood.
Now lambs and negroes both are harmless things,
And thence perhaps this wondrous virtue springs.
'Tis in the blood of innocence alone--
Good cause why planters never try their own.

William Cowper

http://www.poemhunter.com/poem/epigram-12/