This is a revised version of a paper in the preprint server with the
same title. The main result has not changed:
We extend cyclic homology from algebras to all schemes over a ring k.
By `extend' we mean that the usual cyclic homology of any commutative
algebra agrees wth the cyclic homology of its corresponding affine scheme.
The change is in the appendix, which is a discussion of
hypercohomology for unbounded cochain complexes of sheaves. We show
that, unlike the bounded below case, the classical (Cartan-Eilenberg)
hypercohomology of an unbounded chain complex is different (!) from the
``hyper-derived'' hypercohomology in the derived category sense.
This has appeared in Proceedings of the AMS, 124 (1996), 1655-1662.