  Using scanning tunneling microscopy, we show that Si deposition on Pt(111) at
300K leads to a network of one-dimensional Si chains. On the bare Pt(111)
surface, the chains, embedded into the Pt surface, are orientated along the
<112>-direction. They disappear within a few hours in ultrahigh vacuum due to
the presence of residual gas. Exposing the chains to different gases
deliberately reveals that CO is largely responsible for the disappearance of
the chains. The chains can be stabilized by a monolayer of hexagonal boron
nitride, which is deposited prior to the Si deposition. The resulting Si chains
are rotated by 30{\deg} with respect to the chains on the bare Pt(111) surface
and survive even an exposure to air for 10 minutes.
