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For an arbitrary similarity type of Boolean Algebras with Operators we dene a class of Sahlqvist identities. Sahlqvist identities have two important properties. First, a Sahlqvist identity is valid in a complex algebra if and only if the underlying relational atom structure satises a rst-order condition which can be eectively read o from the syntactic form of the identity. Second, and as a consequence of the rst property, Sahlqvist identities are canonical, that is, their validity is preserved under taking canonical embedding algebras. Taken together, these properties imply that results about a Sahlqvist variety V van be obtained by reasoning in the elementary class of canonical structures of algebras in V. We give an example of this strategy in the variety of Cylindric Algebras: we show that an important identity called Henkin's equation is equivalent to a simpler identity that uses only one variable. We give a conceptually simple proof by showing that the rst-order correspondents of these two equations are equivalent over the class of cylindric atom structures.

Sahlqvist's theorem for boolean algebras with operators with an application to cylindric algebras