From www.napoleon.org/en/gallery/pictures/files/dress_expo_04; enlarged one quarter by gogm.
This ensemble has the hallmarks of Empure dress - a ruff at the neckline, short round puffed sleeves, a high ("Empire") waistline, close skirt, and train attached to the waist band.
Keywords: 1804, French, scoop neckline, neckline ruff, quarter length puffed sleeves, waist band, close skirt, train