T HE purpose of this volume is to provide a clear account of the general experimental and logical basis of recent atomic theories. CO!lve"rsations with students prove that many who are familiar with atomic models fail to appreciate the experimental background, but that, when explanations are offered, no difficulty exists in arousing interest. Some time ago, the author read a paper entitled" Atoms" before a Society composed mainly of students, and the measure of success which, in spite of difficulties, attended this effort, encouraged him to believe that something further ll1ight be attempted with advantage. The result is embodied in the present \vork, \vhere very little previous knowledge is assumed. The argument goes back to Dalton's theory, from which point the subject is carefully reasoned, the relevant experimental work being quoted at each step. Attention has been specially paid to certain points which, though evidently essential to the argument, appear to have been insufficiently emphasised hitherto.