  Previous work has axiomatised the cardinality operation in relation algebras,
which counts the number of edges of an unweighted graph. We generalise the
cardinality axioms to Stone relation algebras, which model weighted graphs, and
study the relationships between various axioms for cardinality. This results in
simpler cardinality axioms also for relation algebras. We give sufficient
conditions for the representability of Stone relation algebras and for Stone
relation algebras to be relation algebras.
