  We discuss recent experiments in which fine particles of the organic
superconductor Rb$_3$C$_{60}$ or the cuprate superconductor
YBa$_2$Cu$_3$O$_{6+x}$ are held in a polystyrene film that is spin-coated on to
a silicon substrate with or without an intervening gold, or another inert
metallic layer. From SQUID magnetisation data for Rb$_3$C$_{60}$ there appears
to be a striking and completely unexpected increase in the superconducting
transition temperature from $30$ to $45$~K, which is ascribed to coupling
between the electrons in the superconductor and vacuum fluctuations in the
electromagnetic field just above the metallic film. We argue that this could be
a non-intrinsic effect associated with the presence of solid oxygen in the
Pyrex sample tube. We suggest that the ferromagnetic SQUID signal observed for
YBa$_2$Cu$_3$O$_{6+x}$ particles in polystyrene could be attributed to
ferromagnetic particles or magnetic clusters of unknown origin.
