Dr. Barker has made major contributions to our knowledge of solutions and dense vapours by his work on the statistical mechanics of molecular interactions, especially of polar molecules. His studies of the effects of correlation between the motions of adjacent molecules have led to his important new "tunnel theory" of liquids, which in many respects constitutes a marked advance on previous "cell" theories. He has also done high-quality experimental work on alcohol solutions, and has contributed to the study of diffusion in graphite, counter-current distribution in liquids, and critical solution phenomena.