X-Ray Identification and Crystal Structures of Clay Minerals

mented, if not .particularly new. Chapter 15, on the boron hydrides, has been almost entirely rewritten by the translators, with curious results. Historically, Syrkin and Dyatkina were among the first authors to revive the bridged structure for diborane and related molecules, and their views on the electronic structure of the bridge are clearly not entirely acceptable to the translators, who give prominence to Pitzer's subsequent ideas, and ignore altogether the extension of the bridge theory to beryllium and aluminium borohydrides. The effect is confusing, and the altered chapter has lost in clarity what it may have gained in modernity. The more theoretical chapters are not quite so good. I t is, of course, wellnigh impossible to do justice to the wave mechanics of atoms and molecules in 140 pages without oversimplifying or passing over the mathematical and physical difficulties. One must admire the ambitious a t tempt of the authors to do this, and much of the fundamental theory--for instance, the exclusion principle and the variation method--is well explained. But surely some mention should have been made of the fixed-nucleus approximation, and the fact tha t the Aufbauprinzip is only an approximation, even for atoms ? Chapters 5 and 6 show up the resonance theory at its worst, though this does not seem to have been the intention of the authors. We now know that the 'unexcited structures' of large aromatic molecules make scarcely any contribution to the ground state; and counting resonance structures is a theoretically unjustifiable procedure. Also, t h e theory of ionic-covalent resonance, in its simple form, has not stood up to recent critical examination, and can no longer be taken very seriously. These last two criticisms, however, are mainly based on work which has been done since the book was written, and can hardly be laid at the door of the authors. Chapter 7 has been added in translation, and is a good chapter in its own right, t-hough, as one might have expected, it is not particularly well integrated into the rest of the book. The printing is good, and the translation intelligent, though it is regrettable t h a t Mulliken's name should have been misspelt every time it is mentioned--more than twenty times. Taken as a whole, the book is an important contribution to the literature, and is to be recommended to everyone interested in molecular structure. H. C. LONGUET-HIGGINS Department of Chemistry University of Manchester England