What audiences now think of as the folk music of Scotland (and Ireland) has diverse sources indeed, with some tunes and songs entering the repertory as a result of what's known as the "folk process," while others were collected, published, and embellished or altered by classically trained musicians at various times. This remarkable disc from the Scots historical-instrument ensemble Concerto Caledonia covers a major chapter in the history of Scottish song -- its commercial flowering in the eighteenth century. You have to sink your teeth into this disc to get a handle on what's going on -- the tracklist gives only the composers of the music, while the song texts list just the text authors, Robert Burns being the most prominent among them. Songs alternate with instrumental pieces, and neither group quite matches up with the listener's expectations. The program gets off to a shocking start with The Slaves Lament, to a text by Burns; it is a plaint in the voice of a Senegalese slave transported to Virginia. "No homespun 'tartanry' here," notes director and annotator David McGuinness dryly. The instrumental pieces are recognizable as Scots fiddle tunes, but the process of committing them to paper and marketing them throughout the British Isles resulted in some interesting changes: it was often Italian musicians, who ruled English musical life for much of the second quarter of the century, who did the adaptations, and McGuinness rightly characterizes Francesco Geminiani's version of The Broom of Cowdenknows as "bizarre" -- the Italian violin ornaments and the little curlicues of the original song just don't quite mesh wavelengths. It's a fascinating oddity. Geminiani's younger successor, Pietro Urbani, had more success. It is to the credit of Concerto Caledonia that the listener is drawn into these stylistic issues. The group's sound is compulsively listenable, with sharp, perfectly intoned playing that never loses sight of the essential lyricism of the music. The singers, soprano Mhairi Lawson and tenor Jamie McDougall, are perfectly attuned to the poetry of the proto-Romantic Burns (maybe it takes Scots to really do Burns right). The version of The Red, Red Rose included here is not the familiar one, but an earlier setting; the concluding Ae fond kiss, however, is well known among Scots folk musicians. Just try to get through it without a lump in your throat! Several releases in recent years have helped bring alive the vibrant musical world of the British Isles in the eighteenth century; this one, while certainly a bit offbeat, is highly recommended both to lovers of Robert Burns and to listeners paying attention to early music's cutting edge.