Bartók: Sonata for violin and piano No. 2 in C MajorHemphill: The Moat and the BridgeHemphill: One AtmosphereThis week on “The Concert,” we return to music by Béla Bartók and Julius Hemphill, listening to another set of 20th century works based on folk music and blues. Less traditional in form, Bartók’s second sonata is in just two movements, rather than the standard three, the first movement full of Schoenbergian chromaticism and the second once again infused with folk dance rhythms. After the Bartók, we hear two pieces by Julius Hemphill, infused with the blues but incorporating classical as well as jazz instruments.