In recent years, there has been renewed interest in the development of form
al languages for describing mereological (part-whole) and topological relat
ionships between objects in space. Typically, the non-logical primitives of
these languages are properties and relations such as 'x is connected' or '
x is a part of y', and the entities over which their variables range are, a
ccordingly, not points, but regions: spatial entities other than regions ar
e admitted, if at all, only as logical constructs of regions. This paper co
nsiders two first-order mereotopological languages, and investigates their
expressive power. It turns out that these languages, notwithstanding the si
mplicity of their primitives, are surprisingly expressive. In particular, i
t is shown that infinitary versions of these languages are adequate to expr
ess (in a sense mode precise below) all topological relations over the doma
in of polygons in the closed plane.