Maurice de Fleury’s short story entitled “Hydrargyre” (1887) tells the tale of two lovers who die intoxicated by the waters of the Seine River, which have turned to mercury. This “poème en prose décadent” challenges the ways in which matter might be aestheticized in literature, all the while raising the question of the aesthetics of the prose itself. Indeed, the thickened and densified fluid embodies a metallic mass which provides stark contrast to the traditionally clear waters, an image used by writers and critics in 19th-century France to characterise the “grand style français”. Here, the material imagination, the phantasmagorical description of the urban landscape, and the stylistic intricacies of the text unite into a single image—that of the Seine. We bring scrutiny to this ambivalent treatment of the Seine, through which the author promotes an aesthetic that is symptomatic of the major literary and stylistic issues of the fin de siècle.