This book aims at giving a general account of the principles of valency and molecular constitlltion, founded on tI1e Rutherforcl-Bohr atom. In dev'eloping the theory of valency there are two courses open to the chemist. He may use symbols ,with no definite physical connotation to express the reacti,rity of the atoms in a molecule, and may leave it to the subseqtlent progress of science to discover what realities these symbols represent: or he may adopt the concepts of atomic physics-electrons, nuclei, and orbits-and try to explain the chemical facts in terms of these. But if he takes the latter course, as is done in this book, he must accept the physical conclusions in full, and must not assign to these entities properties which the physicists have found them not to possess: he must not use the terminology of pl1ysics unless he is prepared to recognize its laws. I have endeavoured to conform to this principle, and not to lay mysetf open to the reproach of an eminent physicist, that when chemists talk about electrons they tlSe a different lang'uage from physicists. I have been careful to avoid as far as possible the introduction of any physical llypotllcses wllich are not already sanctioned by those who are best qualified to judge of them.