The Bakerian Lecture, 1962 The structure of liquids

A satisfactory picture of the structure of liquids has lagged far behind that of other states of matter. Ever since the time of Euler in the eighteenth century or, in a more precise form, since that of Maxwell in the nineteenth, we have had a convincing qualitative and quantitative picture of the chaos that is represented by the movements of the ideal gas molecules. The notion of a crystal or a solid in general as an arrangement of molecules ‘ in rank and file’, as Newton put it, is, in fact, older than Newton yet its quantitative statement was made possible only through the work of Born and others in our own century. But it is admitted even by those who work most in the field that the study of the structure of liquids or any exposition of their properties in atomic terms is still largely to be sought. This is not for want of trying. A vast number of researches have been devoted to attempts to analyze the structure of liquids, either directly by the diffraction methods which have proved so successful in crystalline solids, or, indirectly, through the construction of models and their thermodynamic testing. But we still lack either an adequate picture of the arrangement of molecules in a liquid or the necessary quantitative theory to explain their thermal and other properties.