Though the nature of the matter embraced has rendered the division into three parts desirable, the whole subject has been treated primarily from the chemical standpoint. In view, however, of the occurrence of considerable quantities of monazite within the British Empire, and of the possibility that in the near future the Brazilian fields will not remain the sole source of thorium nitrate, stress has been laid on the technical aspect, which is more especially developed as regards the production of monazite and the incandescent mantle industry in Chapters VII and XVII-XX. In the preparation of Part I full use has been made of Dana’s indispensable ‘System of Mineralogy,’ as well as of the encyclopædic ‘Handbuch’ of Hintze, whilst for Part II the excellent monograph of R. J. Meyer, in Abegg’s ‘Handbuch,’ Vol. III, Div. I, and the work of the same author and Hauser, ‘Die Analyse der seltenen Erden und der Erdsäuren,’ Vols. XIV-XV of ‘Die Chemische Analyse,’ have been of service. I have great pleasure in expressing my gratitude to Mr. A. Hutchinson, of Pembroke College, Cambridge, who has kindly read for me the manuscript of Part I, and suggested improvements; to Dr. H. J. H. Fenton, of Christ’s College, who has given me similar assistance in Part II; and to Dr. S. Ruhemann, of Gonville and Caius College, who has read Parts II and III. I am also greatly indebted to Mr. E. J. Holmyard, of Sidney Sussex College, who helped me with the preparation of Part II; and to Mr. H. M. Spiers, of Gonville and Caius College, who read the proofs for me with special thoroughness and care. I have also to thank Professor Soddy and his publishers, Messrs. Longmans, Green & Co., for kind permission to reproduce from ‘The Chemistry of the Radio-Elements’ the diagram on p. 138. S. I. LEVY. CONTENTS PART I _OCCURRENCE OF THE RARE EARTHS_ CHAPTER PAGE I. THE NATURE OF THE MINERALS AND THEIR MODE OF OCCURRENCE 1 II. THE SILICATES 30 (_a_) _Silicates of Yttrium and Cerium Metals_--Cerite; Gadolinite, Glowing of Minerals; Allanite, Hellandite, Thalénite and Thortveitite; etc. (_b_) _Silicates of Thorium and Zirconium_--Thorite, Zircon, Naegite; etc. (_c_) _Mixed Silicates_--Eudialyte, Beckelite; etc. III. THE TITANO-SILICATES AND TITANATES 52 (_a_) _Titano-silicates_--Yttrotitanite, Titanite; etc. (_b_) _Titanates_--Yttrocrasite, Delorenzite, Ilmenite; etc. IV. THE TANTALO-COLUMBATES 60 (_a_) _Containing no Titanium Dioxide_--Samarskite (Annerödite), Plumboniobite, Yttrotantalite, Fergusonite, Sipylite; etc.