Really, mercury engines were already patented as early as 1851 by Michael Faraday, however using mercury alone wouldn't have worked using the technology they had back then.
https://m.youtube.com/watch?v=KqvthexkLN0
https://m.youtube.com/watch?v=kcRo5xvEkdM
https://m.youtube.com/watch?v=iWamgi1Mu1Y
Xerum 525 is more credible than you think because BepiColombo and its orbiters, the MPO and Mio, which study Mercury's plasma and magnetic environment, or the VASIMR engine, a type of plasma propulsion system being developed for potential future space travel. Mercury's own magnetic field interacts with solar plasma, a key aspect studied by these probes.
When you start thinking in terms of plasma physics and magnetohydrodynamics (MHD), the idea of a mercury-based plasma interacting with strong EM fields isn’t inherently impossible. It’s just extremely difficult to control.
Mercury vapor can be ionized into plasma relatively easily. Mercury-vapor lamps are a mundane example. Ionized mercury has both charged particles and a magnetic moment (for Hg-199/Hg-201), meaning it will interact strongly with magnetic and electric fields. This is exactly what probes like BepiColombo’s MPO and Mio study on Mercury in how charged particles in the solar wind interact with a magnetic field and how plasma behaves in a dynamically moving, magnetized environment. So, from a plasma-interaction standpoint, Xerum 525-like fluids could plausibly produce unusual electromagnetic phenomena, if someone could spin, ionize, and contain them.
