E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
A saucepan containing hot water into which a smaller saucepan is plunged, either to keep it hot, or that it may boil without burning. A glue pot is a good example. Mons. Bouillet says, Ainsi appelé du nom de linventeur (Balneum Mariæ). But derivations from proper names require authentication.