This is "A Torrent in a Rocky Gorge" by Achille-Etna Michallon, painted around 1818-21. You can see the foaming water rushing over rocks, a theme that Michallon frequently explored in his work. He's used rectangular patches of colour to suggest the rock face, and quick touches of white paint to capture the swift-moving water.
Look at how he's combined green and blue tones to create a sense of depth and movement in the painting. The paper laid on canvas medium gives it a unique texture, don't you think? Michallon often painted both in France and Italy, but you can see that his work was influenced by the Italian landscapes. The rocky grottoes and crags he depicted are quite characteristic of his style.
A Torrent in a Rocky Gorge hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.