The mysterious group of substances to which have been given the title of “rare earths” has long been the subject of my special study, and no one knows better the magnitude of the difficulties encountered in the investigation, or realises more clearly the comparative insignificance of the knowledge we have acquired. The rare earths constitute the most striking example of the association of chemical substances with others which are closely allied to themselves, and from which they are separable only with extreme difficulty. They form a group to themselves, sharply demarcated from the other elements, and it is my belief that by following the study of them to the utmost limits, we may arrive at the explanation of what the chemical elements really are and how they originated, and discover the reasons for their properties and mutual relations. When this knowledge has been wrested from Nature chemistry will be established upon an entirely new basis. We shall be set free from the need for experiment, knowing _a priori_ what the result of each and every experiment must be; and our knowledge then will as much transcend our present scientific systems as the knowledge of the skilled mathematician of the present day exceeds that of primitive man, counting upon his fingers. The great problem of the nature and genesis of the elements is approaching solution, and when the consummation is reached it will undoubtedly be found that the study of the rare earths has been an important factor in bringing it about. There has long been a need for a work in the English language dealing historically and descriptively with these substances, and Mr. Levy’s book is well fitted to fill the gap. The chapters on the technical applications of the rare earths are particularly valuable, and the chemical aspect of the incandescent lighting industry is admirably treated. The author is to be congratulated upon having successfully achieved an important and useful piece of work. WILLIAM CROOKES. _December 1914._ THE RARE EARTHS PART I OCCURRENCE OF THE RARE EARTHS CHAPTER I THE NATURE OF THE MINERALS AND THEIR MODE OF OCCURRENCE