Barcelona (Freddie Mercury and Montserrat Caballé song) contained "Exercises in Free Love" from "The Freddie Mercury Album". The 1992 version of the 7" Spanish record had another version of "Barcelona" on the B-side, and the rare 1987 12" Hong Kong record was one-sided and had a unique sleeve. Most 5" CDs contained two or three versions of "Barcelona" and "Exercises in Free Love". Most 7" record and 5" CD covers featured Mercury and Caballé singing on a stage with an orchestra on the background, though the Portuguese version pictured them in a static studio photo. The 1992 reissue 7" records contained another version of the singing artists,
Mercury in fish plant emissions. Chlorine chemical plants use mercury to extract chlorine from salt, which in many parts of the world is discharged as mercury compounds in waste water, though this process has been largely replaced by the more economically viable membrane cell process, which does not use mercury. Coal contains mercury as a natural contaminant. When it is fired for electricity generation, the mercury is released as smoke into the atmosphere. Most of this mercury pollution can be eliminated if pollution-control devices are installed. Mercury in the United States frequently comes from power plants, which release about 50% of the nation's