Eloquence is logic on fire.

Lyman Beecher (1775-1863) American minister, preacher, abolitionist

(Attributed)

This phrase is widely attributed to Beecher, but I cannot find a primary source. It is possible the overall phrase is from a combination of different Beecher comments about Theodore Weld (1803-1895), one of the founders of the American abolitionist movement. Beecher one time described Weld being "as eloquent as an angel, and as powerful as thunder." More significantly, James Monroe, an Oberlin faculty member, mentioned Beecher and Ward in a lecture about early abolitionists (published in 1897):

I was never so fortunate as to hear Theodore D. Weld ; but I constantly met those who had heard him, and all reports justified Dr. Lyman Beecher's description of his eloquence as "logic on fire."

Both or just the latter reference could have synthesized the attributed full Beecher quote, pairing "eloquence" to an actual phrase of Beecher's.

In 1848, the phrase "logic on fire," attributed to Beecher, shows up.

In 1881, it's recalled that "Set your logic on fire" was a common advice of Beecher's.

Throughout the latter half of the 19th Century, the phrase "logic on fire" is applied to the speaking ability of various preachers and orators, often without reference to Beecher, but, when it is attributed, it is to.

In summary, while "logic on fire" when speaking of eloquence or speaking style appears to be Beecher's (primary source unknown), the actual phrase starting with "Eloquence is" cannot be pinnned down as clearly.