This lament of Sir William Crookes which introduced the preface to the first four volumes of this series still beckons hauntingly for satisfaction. Although many of
the greatest mysteries he contemplated have been satisfactorily solved, there still
remains the need for voyages of discovery on the "unknown sea".
We anticipated the need for other volumes to follow the first four, both to cover
topics already mature but not included and others whose time had not yet come. We
underestimated both aspects of this need. There is a time for a review of past work
when the review itself promotes understanding and stimulates new lines of re-
search. In response to this the publisher will continue the series to assure an evolv-
ing authoritative and comprehensive Handbook on the Physics and Chemistry of
Rare Earths. Volume 5 contains new topics concerning thin films of alloys and
compounds, transport properties of intermetallic compounds, catalytic behavior of
metallic and non-metallic surfaces, defects and transformations in oxides, physical
and chemical properties of fluorides, and spectroscopic properties of lanthanide
ions in a variety of ionic hosts.