It's a Nocturne, but I don't remember its name. Isn't it the one that Ruskin was so rude about that Whistler sued him? I saw the painting at a brilliant exhibition at the Tate Britain a number of years ago which featured Whistler with Turner and Monet.
Nocturne in Black and Gold: the Falling Rocket. It lives in the Detroit art museum and I have seen it. It is smaller than I expected! The quote from Ruskin was that "Whistler has thrown a pot of paint in the face of the public."