It was to have served as a museum of natural history, but after some eventful delays (Napoleon's invasion badly damaged the building), its mission was diverted to art, and the royal museum of painting was inaugurated in 1819.
It was initially planned to be a different type of museum but eventual became an art museum.  
mnli hypothesis: It was initially planned to be a different type of museum but eventual became an art museum.   premise: It was to have served as a museum of natural history, but after some eventful delays (Napoleon's invasion badly damaged the building), its mission was diverted to art, and the royal museum of painting was inaugurated in 1819.