This is one of my favorite BML myths!
The following paragraph from the 1771 De Brahm’s Report of the General Survey in the Southern District of North America is often refenced as the definitive earliest written European document verifying the existence of the BMLs.
"Although these Mountains transpire through their Tops sulphurueaous and arsenical Sublimations, yet they are too light, as to precipitate so near their Sublimitories, but are carried away by the Winds to distant Regions. In a heavy Atmosphere, the nitrous Vapours are swallowed up through the Spiraculs of the Mountains, and thus the Country is cleared from their Corrosion; when the Atmosphere is light, these nitrous Vapours rise up to the arsenical and sulphureous (subliming through the Expiraculs of the Mountains), and when they meet with each other in Contact, the Niter inflames, vulgurates and detonates, whence the frequent Thunders, in which a most votalized Spirit of Niter ascends to purify and inspire the upper Air, and a phlogiston Regeneratum (the metallic Seed) descends to impregnate the Bowels the Earth; and as all these Mountains form so many warm Athanors which draw and absorb, especially in foggy Seasons, all corrosive Effluvia along with the heavy Air through the Registers (Spiracles) and thus cease not from that Perpetual Circulation of the Air, corroding Vapours are no sooner raised, than that they are immediately disposed of, consequently the Air in the Appalachian Mountains in extreamely pure and healthy."
References to this passage are misguided, misquoted and/or taken out of context. In this passage, De Brahm was writing about the origin of thunder and lightning and the health benefits of breathing the pure ‘Mountain Vapors’ of upstate South Carolina! The passage occurs in the South Carolina chapter of his book and De Brahm does not even describe North Carolina anywhere in his 325 page book---he may have never even visited North Carolina! The influence on the health of the local inhabitants by breathing good air versus bad air is a repeated theme throughout De Brahm’s book, and elsewhere he describes, with similar floury words, the poor health effects of breathing ‘Pond Vapors’ (p. 79) and ‘Swamp Vapors’ (p. 160) and even goes on to suggest “a prudent and moderate use of Spirits” to offset the effects of living in an area where one is constantly breathing bad air.
The first reference to De Brahm’s 1771 report can be found in R.K. Babington's 1927 article in The Gastonia Daily Gazette; however Babington failed to reference his source--so it is unclear if he was quoting someone else or if he had actually read De Brahm's report, which in 1927 was still a very-difficult to read hand-written document. It also appears that many later authors picked up the story without properly checking their references either.
Refreshingly, Brian Dunning, in his 2010 podcast on Skeptoid.com comments:
"Taken in context, it's clear that de Brahm's quote has nothing wahtsoever to do with the Brown Mountain Lights. This leaves us with no documentary evidence that the Lights existed prior to the arrival of electric lights and people in the area in the early 1900's."