(QUITE RED IN THE FACE) Yike. Just goes to show that some of the most basic things are forgotten when not used. Now I remember. And a clarification - the atomic weights of elements (as mentioned to me several times now) are C-12 based. They used to be O-16 based (I suppose before 1969), which may be why Avogadro's number changed, as well. Scott Schwahn (embarrassed to use title) schwahn@cebaf.gov