  Dielectric measurements at very low temperature indicate that in a glass with
the eutectic composition BaO-Al$_2$O$_3$-SiO$_2$ a phase transition occurs at
5.84 mK. Below that temperature small magnetic fields of the order of 10 $\mu$T
cause noticeable changes of the dielectric constant although the glass is
insensitive to fields up to 20 T above 10 mK. The experimental findings may be
interpreted as the signature of the formation of a new phase in which many
tunneling systems perform a coherent motion resulting in a macroscopic wave
function.
