THE progress of atomic and molecular theory in recent times has proved remarkably rapid and extensive. The subject has advanced along many different lines more or less independently, and the research worker in a specialized branch often encounters no little difficulty in keeping in touch with developments not in his immediate sphere of thought. It has become increasingly difficult, though surely not less important, to correlate work carried out in different branches. It is in the hope that these diverse researches may be enabled to be considered more readily in inter-relation with each other and in respect of the effect of each contribution to the whole that the author has essayed the present task. A work which attempts to view the situation along various avenues of approach may perhaps also proye of value in enabling a given fact or principle to be found conveniently in the original literature.